Now for headphones...
Keep in mind I am accustomed to my HD580s, as I've had them for about three years, so I a little biased to their sound and something that sounds brighter or more forward to me may not be the same for others...
HD650 - Heard these before, I'm still not really impressed with these, they sound dark and thick and I don't really think they are worth upgrading to.
HD555 - owned these before, and sold them, I thought that was a dumb move now that I got to hear them again. They sound quite different from what I remember, they sound similar to the HD580s except richer and forward in the midrange (especially lower mids), they are lacking a little in resolution and sound ever so slightly grainy, with the same rolled off highs. I am actually impressed with these cans, although at their retail price and the fact they need an amp to sound good, you are probably better off getting the hd580 anyway. Which brings me to....
HD595 - now these are how I remember my hd555 sounding like. They are more neutral than the 555 and have less of a midrange hump. But the mids are seriously grainy here and this ruins the sound imo. I hear the same thing with the stock cable and also with the pair with the special cardas upgrade. Midrange performance is very important to me so a can that does poorly in this area is a real letdown. Get the hd555 instead.
HD201 - Now these are fun cans! Very colored but still highly enjoyable. They have a dark, smooth, liquid sound and the midrange is good without the flaws that plague most cheap headphones. These are Senns answer to the grado sr60 and i think they are even better than those. Hell I'm going to get a pair for general use myself!
EH-350 - a decent phone, a little bright and uncomfortable but otherwise ok. Would be a good budget phone until I saw the $79 price tag, which is a little steep for these which have worse build quality than the older and cheaper hd497
hd485 - good bass, and good comfort, but a bit muffled sounding. Would probably make good gaming cans.
hd280pro - Honky mids and really rolled off highs...not my cup of tea at all
These are all Sennheiser Cans...now for the non-senn stuff:
AKG 501 - Lacks bass and sounds a bit aggressive in the upper midrange. The highs seem a little uneven too. Overall they have an unbalanced sound but they are ok for some music (acoustic and jazz) but not so good for others.
Ultrasone HFI-550 - a closed headphone? A bit pricey for what you get at $189, but a pretty decent sounding phone. Fairly well balanced. Would complement the HD580s well if someone is looking for a closed phone to get. Comfortable too.
Bose Triports - Yuck! Very poor performance for this price. They are comfortable though - but they sound worse then most $30 headphones! Heck I dont think they sound any better than my old Jensen cans
Didn't listen to any grado phones as I don't care for the grado sound..
Didn't get to listen to the DT231 or the other Beyer phones..
Didn't want to listen to the CD3000 as I've heard that POS enough times
The only headphones that were interesting to me were the hd555, hd201, and hfi-550.
The amps were much more interesting...