So, without further padding, let's get to it!
First I want to talk about the packaging of these two, because it's different enough to warrant a bit of discussion. Grado headphones notoriously come with fairly spartan packaging - in the case of the RS-1 you get an all-white box with a sheet of paper adhered to the front of it that lets you know what's inside, and then a foam insert to hold the headphones and their included accessories (the Grado extension cable and Grado adapter). It's fairly plain, but I wouldn't call it inappropriate - they're very straightforwardly presented, and my set came undamaged and in pristine shape from the retailer. By contrast, the ESP/950s include the better part of a Radio Shack in the package! First you're presented with a carry bag, and then inside that you have the headphones, their amplifier, extension cable, adapters, interconnects, a battery pack, and the user's manual which explains how you use all of these accessories and bits. The 950s are the first headphones I've seen that also have "some assembly required" - the ear-pieces detach from the headband, and must be installed for use (this is by design, and they are meant to come apart like this to travel; I think the idea is to make them more durable, I don't know, and I haven't taken my set mobile).
When it comes to hooking these sets up, the RS-1 are very straightforward; just plug them into whatever headphone jack/device you have (headphone amp, mp3 player, whatever). The ESP/950 however are designed to plug into their E/90 amplifier, which itself plugs into a line source via RCA (cables *are* included), or TRS (cables *are* included). They mention using a portable CD player in the manual, but I have neither tried that nor an mp3 player. I know that with most older PCDPs I've seen, they feature both a headphone out and a line out, so it's entirely possible that the E/90 would be an ideal candidate for an LOD if you're going to use this set mobile (it does include a battery pack and all of this will fit in the carry-case; it uses 6 C-Cell batteries at a time and gives no indication of estimated lifetime). The RS-1, by contrast, are easily driven by most devices (like other Grados) and you can very likely take them out with your iPod and no other supporting hardware. That's about the extent of my insights on these as mobile cans, as I use both of them exclusively at home.
Onto the sound!
In my early impressions of the RS-1 against the ESP/950 I made a statement along the lines of "the ESP/950 is like the teacher's pet and the RS-1 the class clown, both will end up with good marks for opposite reasons." I would have to say that I still agree with that sentiment. The biggest contrast between these two is the ESP/950's "technicalness" and the RS-1's lack thereof. The 950s are more than happy to show off exactly how fast, smooth, detailed, accurate, etc that they are. The RS-1, by contrast, are content to just do their own thing and go with the flow. But that's boring, so let's talk about the sound some more. I like the format of talking about the mids/bass/treble individually and then providing overall impressions after that, so I'm going to do that here. In all cases these impressions are relative to one another, not necessarily absolute against the world.
In my opinion, the reason to listen to music. Strings, vocals, guitars, pianos, and so on. It's where it's at. Between the two cans, I would say that the ESP/950 are more consistent in their midrange presentation; the RS-1 are a bit more of a wildcard. If you've got a track with stellar voicing or a great string section, both cans will absolutely sing here. But on the other hand, if you have something relatively lackluster, the ESP/950 are probably your choice - they will still "calm down" or "withdraw" but it's not as dramatic as the RS-1. They still impress with their ability to hit any nail on the head, but the RS-1 just pack up and go home. Vocals are absolutely beautiful through either can, but the 950s probably get the nod in "accuracy" (depending on how you define that) - they don't tilt or favor any specific vocal presentation or style. They're as good with Sinatra as they are with Girls' Generation. The RS-1 seem more selective; some vocalists "jive" with them, and others do not. It really just depends on the recording in question - most all of the recordings that I consider to have spectacular vocals, come through just beautifully on the RS-1. With those that are less than perfect, it's bit more erratic.
One thing the RS-1 does more naturally than perhaps any other headphone I've yet heard (including the 950), is relaying "pub noise" (like the intro of Michael Jackson's Black & White) - it sounds incredibly realistic through the RS-1, scarily so. This carries into their vocal presentation with videogames and well-done recordings; if everything is spot-on, they're among the most correct sounding presentation of the human voice out there. The 950 are probably 99% of this, but will not "pack up and go home" if conditions aren't perfect. Neither of them is as brutally unforgiving of BAD vocal recordings as the MDR-F1 (by contrast), so that's also a plus.
I'll start this out by saying neither of these cans will shatter your head with bass, but what they will do is provide very clean and controlled low-end response. The RS-1 have a more satisfying impact here, but lack a bit of the absolute extension that the 950s provide. With techno/electro, the RS-1 are highly satisfying and provide enough low-end impact to really let the bass-line drive the music. The 950s are competent in this regard, but are not able to be "driven" by the bass if that makes any sense. The expression related to the mids "get out of the way of the music" seems more applicable to the 950's bass - they can reproduce bass probably better than any speaker/sub system I've heard, and better than most headphones, but they don't shove it down your throat (even if the material calls for it). The RS-1, by contrast, are able to produce enough of a low-end thud to make bassy music more engaging.
Both offer a very controlled low-end, the 950 retaining their signature "effortless" presentation, and the RS-1 offering more impact. The RS-1 are able to texture the bass presentation very nicely, while the 950s will only bring through what the material originally offers. So if you're listening to a very dry drum machine, it can sound somewhat ho-hum through the 950.
With videogames that feature lots of explosions, both are able to relay that, but neither rattles your head with it. I think both are very suitable here, and for bassier music, but if you're a "basshead" I'm skeptical that either of these will provide the excessive low-frequency output you crave. That having been said, I really do enjoy the RS-1 with Zombie Nation, VAC, and various electronic compilation albums, and I find the ESP/950 quite enjoyable with hip-hop and rap (much like the Sennheiser HD 580/600).
Here's where it gets a bit more interesting between the two. Similar to the bass discussion - the 950 offer extension, and the RS-1 offer impact. The RS-1, however, are not harsh or sibilant, even with nasty material; they don't offer the smoothness of the 950s though (one great thing about the 950s is that even very clashy tracks are not painful to listen to). The RS-1 are truly an example of bright done right - brass, woodwinds, and synth noises are a joy to listen to, not a punishment. This is not to say the ESP/950 are not ridiculously good up here - they extend the treble very far, and can hit very high notes without being screechy or painful (unless you turn them up too loud), and represent perhaps the best treble I've heard in a headphone.
Both have enough energy up here to make cymbals, guitars, bells, and so on very convincing. I know there's measurements that show the RS-1 being something like 10-15 dB (seems to depend on who was testing) up at 10k, and I don't doubt that, but they are not aggressive/forward/harsh like most headphones that are that boosted up. They also don't degrade into the cold or analytical sound that the other "half" do. The ESP/950 by contrast are not emphasized in this region, instead they just possess the prowess to extend very well and carry a note without burning it into the back of your eyes.
I'd like to start here by saying that most of the generalizations about Grado's having a closed-in or congested or otherwise tiny stage are probably based on the lesser models. I certainly feel that the SR-60 or SR-225 for example possess these traits. However the RS-1 do not. They are able to present a very convincing 3D image that encapsulates your entire noggin and can place performers, instruments, sound effects, or whatever else anywhere in that sphere. The "catch" (if you will) to this very coherent imaging, is that the sphere is relatively small. What separates them from the rest of the pack is that they provide great bass impact while providing this good stage - most headphones with a very coherent 3D soundstage lack any meaningful extension or impact in my experience.
The 950s are a bit of a contrast to this - they are equally coherent, but they can image much larger. Think about the difference between balcony seats and sititng at the sound board; the ESP/950s are up in the balcony. You can pick the notes out of the air at your leisure. However they are not as "weird" as the K701 (which make everything into a grand orchestral production) - they can present very close-up images and resolve very minor details (like singers catching their breath, or brass performers doing the same). The RS-1 don't seem as able to "move about the room" - they're not able to hit that very furthest distance (which it doesn't seem many recordings really require, at least modern music).
With videogames, I was somewhat hesitant to try the RS-1 out, because I never had a good experience with the SR-225. However they are an absolute delight - they can accurately place characters and effects on-screen and will gladly show you if someone is sneaking up behind you. They aren't "the best" at this (the MDR-F1 are my standing favorite for soundstaging), but they certainly hold their own among other hi-fi headphones. The 950s are superior in this regard though, the "effortless" presentation combined with their larger (at will) stage makes them better able to present a convincing image of a large game world (like you might expect in something like Halo). I believe that gamers who are also audiophiles would find either headphone perfectly acceptable, but I don't think competitive gamers who don't care about hi-fi music reproduction should probably look elsewhere (I think you can get equal or better performance for less money (for example the Ultrasone HFI-2400 is a very fun/good headphone for gaming, and it costs around $200).
More direct comparisons
Overall I think these two are both very good at presenting a convincing music listening experience. They aren't missing any major range of the frequency spectrum, they don't do anything terribly offensive or inappropriate at any point, and they handle a wide range of genres with ease. That having been said, I feel that there are some fundamental differences that characterize them as very different headphones. The RS-1 are much better at bringing out emotion or feeling within a piece of music, and letting the mids or the bass absolutely drive the presentation, while the 950s are better at bringing out the details and presenting the entire piece "as is" - letting the user digest it at leisure. The RS-1's bring you to the music, the ESP/950 bring the music to you. With the ESP/950s music becomes a more interpretive experience - you're presented with everything, in a relatively even-handed manner (they're perhaps slightly mids-tilted/warm), and you can focus on whatever portion you like. The RS-1 will, by contrast, "latch" on a component of a piece and let that drive the entire experience. Neither is "wrong" but it's the most fundamental difference I've picked up between the two cans - as un-technical as it is.
A word on comfort - the 950s are more comfortable headphones. There, I said it. Having said that, I don't want to say the RS-1 are uncomfortable. They are on-ear, which is a turn-off for a lot of people, but they don't seem to suffer from any of the plagues of on-ear. They don't cut into your head, they don't feel overly heavy, and they don't heat your ears up (in fact, the foam pads absolutely live up to their reputation of not heating up and being a great choice in summer heat). I think both are very livable, but if you don't like noticing that you're wearing headphones, the ESP/950 are probably a better choice for you (they really feel weightless, despite weighing more than the RS-1).
Durability - I don't think either of these will survive a lot of abuse. The 950s are mostly plastic, and while plastic can be resilient, it also can snap or break if subjected to a lot of stress. I think if you accidentally sat down on or dropped (from a great distance, like off a balcony or something) them they would probably crack somewhere. They feel sturdy enough for normal at-home use though, and there's no creaking or snapping when they're adjusted. The RS-1 by contrast are more of a metal/wood affair (the cups are wood, the head-band, rods, and whatever you call the thing that attaches the cups to the rods, are metal). They are perhaps slightly more resilient as a result, but remember that wood can chip, pit, and crack relatively easily, and I don't think these would very much enjoy being sat on. Both of them are very much "for home" headphones (I'd also be somewhat worried about exposing the RS-1's cups to a lot of direct sunlight, as this can damage some wood finishes and will age leather (the headband is leather) - I do not know if this is the case with these, just a general observation).
The cables on both feel high quality, and I'm not worried about them breaking or coming apart through normal use - they also have nice feeling connectors for plugging into their respective equipment. The Grado 1/4" to 1/8" adapter is fairly robust as well - the 1/4" side grips the plug fairly aggressively, which is nice - I'm not worried about pulling the adapter out and can treat it as basically an extension of the factory cable. I haven't tried either extension cable, so I'm not sure how robust those are, but they both appear to be made from the same exact cabling and connectors as the headphones have hooked up to them. I really don't see a need for either though (I like that both of these cans have less than 6 ft of cable from the headphones to the plug - and they both include extensions if that isn't enough for you; I think the Koss extension is right around 6ft, and the Grado cable is closer to 15ft).
Replacement Parts - Both manufacturers seem to have a commitment to longevity of their products in the field, and offer a lot of user-replaceable parts at a decent price. Koss wins in terms of absolute pricing (earpads for the ESP/950 are $5 a pair!), but neither of them gouges up spares (none of this $150/set earpads or $200 headbands). Both make pads and replacement accessories (adapters, extensions, etc) available for purchase. With the Koss, any other damage/issue is solved by their Lifetime Warranty, while Grado seems to be willing to perform repairs on the headphones (maybe for a fee? I don't know - they've always been responsive to contact though, and I know they will re-cable your headphones among other services, so I don't think it would be unrealistic to contact them if your set is damaged, even out of the warranty period). I think in both cases, the spares pricing is fair, given the initial purchase price and overall packaging of the cans - I personally find it annoying when you buy a $300-$400 headphone and spare pads will run you $100 or more.
If you need spares, you can buy the Koss components by calling Koss' 1-800 number (I think the earpads are available online though), and Grado spares come from an authorized dealer.
On a related tangent - I have no idea about the Koss' carrying bag. I've seen some questions in the past about whether or not that bag is covered under the lifetime warranty, and people joking about sending just the bag in with an "explanation of the problem" (as per Koss' warranty terms) and seeing if they could get it repaired or replaced. I would believe that Koss would probably help in some way if that bag were damaged, and there isn't an explicit exclusion of that bag in the users manual (at least that I've found), but it's still somewhat curious to think about. The bag is, unfortunately, not sold separately (at least not that I've seen online, and I haven't thought to call and ask if I could buy one - it'd make a nice gear bag in general). I keep mine in the original box (since I don't take the gear out on the road), but my 10 or so minutes of interacting with it did inspire confidence in it's ability to protect it's cargo and stand-up to reasonable use. If anyone has trashed one of these bags and gotten Koss to replace it or repair it, I'd love to hear about it, or any other information about the whole thing would be interesting! (afaik this is the only production headphone to include a bag like this - it goes way beyond the "hard case" that you'll see with something like the Bose QC15 or Ultrasone PROlines, but isn't artsy/display material like the Ultrasone Editions).
Isolation/leakage - There is absolutely none on either can. However the RS-1's seem to leak a lot worse than the 950s (even at really low levels, the RS-1s leak out a lot of noise, while the 950s are pretty quiet). Neither of these are suitable in a noisy environment.
Looks - A bit more personal and perhaps irrelevant, but I think that the RS-1 are probably the more handsome of the two. The wood finish isn't what I'd call heirloom quality, but it's certainly very good (better than the ATs I've seen), and they have a bit of personality as a result of looking rather different from most other headphones. The 950s look neato for sure, but they're sort of plain-jane sitting next to the exotic RS-1.
Break-down of the above (aka if you forced me to give ratings):
Overall Extension: ESP/950
Overall Impact: RS-1
Soundstage depth/width: ESP/950
Soundstage coherence/accuracy: Toss-up
Value-for-money: RS-1 (the 950s recently jumped up to MSRP price-wise, so they cost about 50% more than the RS-1)
And that's where it's hard to declare a true winner. I really enjoy spending time with both cans, and it's hard to declare one absolutely better than the other. Depending on my current mood or the track I'm listening to (or both), I might find one more enjoyable than the other, but I don't feel that the 950 outright trounces the RS-1 (which is probably a first; if you've read my previous posts, you know I'm a big proponent of the ESP/950 as the best headphone ever made). I think both companies did a great job of designing some very competent and very enjoyable headphones, the RS-1s being potentially the best dynamic cans I've ever sampled.
If anyone has questions, or feels like I didn't cover anything, feel free to ask. I have both of these on-hand, and would be glad to answer.
Finally, a note on music/material used:
I don't like A/Bing with a specific track-list or similar, so I just use whatever can until I either get disgusted by it (and chuck it), or get a good feeling for it. In this case, both of these have just been used for my normal music listening and gaming sessions, so whatever I'm listening to, they get to play. My music tastes are pretty diverse, but I'll list off some artists that aren't uncommon to see in rotation:
The Notorious B.I.G.
Games wise, I don't play extremely competitive games anymore, so I'll usually be rotating RPGs or RTSs, with a light smattering of FPS titles, like:
Fallout 3 and New Vegas
The Elder Scrolls series
Various "Sim" titles (like SimCity)
Total War titles
On equipment used, see my profile for more information.
Like I said, it's pretty much all over the place, so I don't try to focus on headphones or speakers that are one-trick ponies, because they don't last very long in this environment. If you have specific questions about how these two stack-up in terms of a given genre or type of music, I'd be happy to answer them, but my intention is more to provide a general over-view of the two.