Finally got some stuff back! All of these are modified, some more than others. All are being driven from a Geek Out 450.
1: Sansui SS-100:
Most transparent by far. Distorts a bit compared to the HE-6 (especially in the upper midrange) but cleaner, smoother, and far more natural sounding. My absolute favorite of anything I've ever tried, the midrange is truly something special and they maintain fantastic extension into both bass and treble. By far the best seductive characteristic of my gear; these are fantastic at sucking you into the music. Really comfy too, the headband cushioning is spot on despite being thin and they don't clamp at all -- they just kinda float there. I did have to switch them to over ear pads to achieve this though. Not very nice feeling stock. I love them, a lot, though not as much when it's loud in my room due to how open they are -- noise cuts through the bass like a knife and makes them sound thin. I usually turn the aquarium filter off when I use these. Not suitable for meets!
2: HiFiMAN HE-6:
A one word descriptor for these would probably be "huge". The way this thing presents its sound is very... huge, I don't know how else to put it. The closest thing was listening to the speaker system in a small auditorium. Their bass is very punchy and these slam harder than anything I've ever heard if you give them the right material. However, they have a slight shelving of the treble and upper midrange -- about -8dB at 1kHz. I'm happier at around the 5dB mark. The imaging also doesn't seem as precise as the Sansui, but I'm still waiting on the speaker amp for these, so that will probably change. Headphones that need extra power have always sounded slightly "cloudy" to me -- likely a side effect of the source equipment not responding well to the load or not pushing enough juice for it. Though, the fact that they already sound as good as they do is very, uh, promising... and almost frightening. A side note, these were modded by someone else, so they're not necessarily entirely to my taste. This also means they use Audeze vegan earpads and the headband is slightly stretched out -- so even despite being the heaviest headphone I've ever held by far, these are still just as comfortable as the Sansui. Well, maybe the pads retain a little more heat...
3: Pioneer Monitor 10-II:
Absolutely my favorite closed headphone and the best I've heard thus far. They provide one of the most amazing bass presentations I've ever heard. According to my measurements, they actually distort less than the HE-6 in the bass and midrange -- yes, really!! The bass distortion is actually so low that it melds into the midrange perfectly and doesn't stand out at all. They also extend very well to bass and treble, which makes for a very complete listening experience. It is very very rare for a dynamic to do this so well! It's actually slightly above the HE-6 in this regard as of writing despite this being an area that planars excel at. They also get to listenable levels out of anything and they can take some serious cranking -- I've had these things up to 120dB and the bass never crapped out. Definitely one of the most unique headphones in my stable. Unfortunately, they are mostly reserved for monitoring and quiet listening, as they have a very sharp spike at 16kHz that rings violently when you turn them up loud. They also have a bizarre characteristic in the upper midrange -- they peak slightly at 1.5kHz (with some ringing) and take a 25dB nosedive into 4kHz. In most cases this is a measurement artifact, with some ringing showing up on the CSD immediately after the dip, but this is not present here. While there is still adequate treble to balance this out, I've heard them as simultaneously sounding slightly dull and shouty on many occasions. Did I mention they're comfy and provide great isolation?
4: KOSS PRO/4AAA:
This beast is unique, the bass is hugely emphasized (almost +15dB) and thick but the midrange and treble, though pretty rough, stay flat and balanced throughout with only the slightest rolloff at the last octave (16-20KHz). The bass can pound the hell out of your skull and they isolate amazingly well due to how the pads are designed -- I've successfully used these as hearing protection while mowing the lawn! While definitely not UNcomfortable for me, they are still the least comfortable headphone here, and they don't quite sound as balanced as the Pioneers; very slightly dark by comparison. They do still resolve well though, which (coupled with the isolation) is why I've held onto them for so long and will continue to do so. One of the more difficult to drive headphones here though; I've had these sound wonderful from one amp and awful from the next.
A couple permanent residents that don't see head time:
5: Pioneer SE-500:
This was the first headphone I bought with my own money and is in very, very rough shape. They were my favorite for a very long time; I always thought there was something "special" to how they sounded. They're modified and have a very nice, clean, lush midrange with bass roll off and good treble extension. Kind of like an electret in that they provide excellent midrange and transparency, but they can be driven from a normal headphone jack. Comfy. Less sensitive than the HE-6... so they're out of use for quite a while. Worth noting, even with the HE-6 and Sansui in the database, they possess the best transient response I've ever measured. So... yeah.
6: BEYER DT48S (5 ohm):
These have an issue with one of the drivers that makes them cut in and out. They were also a gift from a close friend. These resolve far better than they should, honestly up to par with the Sansui in this regard, but the FR is extremely nasty and the distortion behaves oddly. Note: some people call the DT48 thin sounding, but they don't really sound that way to me as long as you get a good seal (which is extremely difficult, but whatever). These are also the only unmodified headphone on this list.
A note: I talked about measurements a lot didn't I? I'll get around to posting some of my squiggly lines soon so you guys can get a look. Right now I just need to find a suitable solution for doing square waves and I'll be good.