I will agree the ATH AD2000s are a good alternative,but try to find the non X version,pretty much a superior version to the X.
We are all making all these suggestions without knowing what style of music you like.Since you have Grados im guessing rock must be part off the mix,but who knows.
I listen to rock/metal/folk 90-95% of the time and for rock I grab my HE-500s,for metal with an aggressive sound I grab my Sextetts(MPs) or AD2000s,theyre essentially a toss up when it comes to metal though the 2000s are a bit smoother,but the Sextetts bring out guitar like no other headphone Ive heard,at the cost of slight grain in upper mids/lower highs.
I used o own the HD600s and while theyre nice headphones I found them rather soulless and bland for my musical tastes.
Note: I said A2000X, not A
D2000 or A
D2000X. Very different headphones. I have not heard either of the ADs, but I do remember the original AD2000 being FOTM some years ago, so there's probably quite a bit written about them, for better or for worse. I have no idea how the new "Z" (there's an A2000Z, I don't know if there's an AD-Z yet) compares sonically, but from what I've read, they dropped the all-magnesium build in lieu of a mostly plastic frame, which is an aesthetic turn off imho. But maybe they sound better, who knows.
For rock and metal I'd agree on the Sennheisers being a bit bland - they just aren't an incredibly fast headphone. Ultrasone and ESP/950 are where I'd be looking then, based on what I've heard, owned, etc. Honestly the ESP/950 were what made me sell the Sennheisers - they do everything the Senns can do, better, and they're *fast*, and (and this is really subjective) more comfortable too.
I have to say that my thoughts about the Grados with the NAD came from something I read about a 1:8 rule of output impedance to headphone impedance. But I've also read that isn't a big issue with many headphones, including Grados... one of the reasons why I bought them. So maybe they don't need a low impedance output - especailly if I am looking for a warmer and smoother sound. I'm actually starting to think that the 10 ohm output of the NAD (which has 2V into 32 ohms) may actually be pretty good for the Grado's.
Short answer is throw that 1:8 thing out with the bathwater. Longer answer is: it's based on some (bad) assumptions about a given transducer or transducer set (this can be a headphone, a speaker, whatever) having some sort of "ideal response" (note how arbitrary that is) wrt its impedance interaction, and that because higher output impedance + more reactive load will result in frequency response variation (from what you ask? the same load with low output impedance source), that "variation" is obviously an error, and therefore higher output impedance is bad. Hopefully this explanation shows exactly how bad the logic here is...
To get deeper:
- The original IEC specification (which is often mocked by people trying to sell something, more on this in a bit) calls for a 120 ohm output impedance, but also for the device to be able to do 5V. A lot of receivers can do this easily, and usually exceed the IEC specified impedance - what they usually are doing is taking a high value resistor (just using equipment I own as a small sample, this can range as high as 560 ohms) and throwing it on the output from the internal amplifier. This is done to bring the level down to something safe for the headphones and the listener.
- This "this is a bad thing" requires the headphones/speaker/object to be a fairly reactive load, that is, the impedance plotted vs frequency has to look like a roller coaster. That + higher output impedance and you get into variations on power transfer and you'll see increases in magnitude at points in the frequency response correlating to the impedance. Not all headphones exhibit this reactivity, for example orthos generally look like a resistor from the amplifier's perspective (e.g. impedance is just flat at whatever its specified at), some dynamic cans are pretty "stable" too (Grado being an example).
- In more recent years, the IEC specification means less and less because a lot of mobile and integrated devices simply can't provide that kind of output, and that's driven a lot of ultra-sensitive, low impedance cans that can get stupid loud with almost no input (e.g. Beats Pro). The point behind those designs is for better compatibility (and battery life) with something like the iPhone.
- Some manufacturers actually have designed, or design, products with the IEC target in mind - AKG, Audio-Technica, and Beyerdynamic are all solid examples of this (there's also similar arguments to be made for speaker design, including by some quite smart folks like Floyd Toole). If I remember correctly, Sennheiser has been ambiguous as to whether or not they care about the IEC target. HOWEVER, this can all boil down to a matter of preference (you aren't risking damage to the headphones) - maybe you don't like to add the flavor packet to your ramen, or you put ketchup in your mac'n'cheese, or you drink your Dr Pepper warm, etc.
So, and here's where a lot of the really hardcore, numbers-driven objectivist guys teeth are going to be grinding, we end up circling back to how we used to talk about amplifiers: try it, listen, hear, and decide if you like how it sounds with the rest of your system.
In the case of Grados, they aren't seeing wild variations in impedance, so you won't get huge variations in frequency response with relative changes in output impedance. This isn't to say with massive changes in output impedance there wouldn't be something happening, but the bigger question is "would you actually hear it?" and "if you could actually hear it, would you consistently notice it?" From experience with various Grados and sources, I will say that for me, the answer is no. Even with Sennheisers and other reactive cans, while I do notice the difference, its pretty easy to forget about if you aren't doing continuous A/B comparisons of gear (and end of the day even with the "big" changes that some Sennheisers can experience, they're still going to sound like they sound, you aren't gonna turn them into something they aren't just by changing the amplifier or source component or whatever).
So, if you want something that's smoother, bassier, etc that's another pair of cans, because you won't turn the Grados into that with another amplifier.
To hopefully prevent a lot of potential screaming: I am not saying all amplifiers sound exactly the same all the time in all usage scenarios. I am not making any absolutist statements. Of the amplifiers I own, and have owned, I would say that to my ears there are slight differences that can, in concert with the rest of the system, either serve to somewhat heighten or diminish enjoyment of the overall setup. Think of it more like adding salt to your food than exchanging your taco for sushi.
Some stuff to read if you really want to get into the whole output impedance discussion:
http://en.goldenears.net/index.php?mid=KB_Columns&document_srl=1389
http://www.innerfidelity.com/content/musings-headphone-amplifier-output-impedance (here, Dr Meier cites Sennheiser as saying the HD 800 is designed with low output impedance in mind, but I vaguely remember reading that the HD 580/600 assumed higher output impedance if for no other reason than they were probably designed before the Cold War officially ended).
I'll also add: anything that goes down the marketing-driven road of "damping factor" or "electrical damping" or whatever else can probably be discarded with prejudice. It's mostly just a rehashing of very old debates.
Some more stuff you might read:
http://roger-russell.com/wire/wire.htm#damping
http://sound.whsites.net/impedanc.htm
http://www.butleraudio.com/damping1.php (I've posted this before and it's cased some confusion - Butler Audio did not write or prepare this article, they're merely hosting an article that was originally written in 1967 by a JBL engineer)
http://eaw.com/amplifier-damping-factor-more-is-better-or-is-it/ (awful grey-on-white text but does a good job exploring how wire loss can muddy this discussion)
http://www.audioholics.com/audio-amplifier/damping-factor-effects-on-system-response
Side note for the peanut gallery: I'm not looking for a fight here, and I'm trying to present this information is as non-biased a way as possible. There *can be* an impact here, but for Grados its fairly small (if noticeable), and for headphones that are more impacted by it, it will ultimately come down to personal preference more than anything else. Yes, something *is* happening, but how you as the listener INTERPRET or EXPERIENCE that something is not easily quantified or objectified.
