Didn't really pay much attention to this thread till now.
Ok, I'll just list the headphones I've owned, and comment on each.
From cheapest to priciest:
Koss KSC-35: For the price, I don't think you'll find anything more balanced than this. It has a very "fun" quality to it which makes it great for portable use. Doesn't handle subtleties like soundstaging too well, but what can you expect at this price. The clips are comfortable, at least for me.
Koss PortaPro: Take the KSC-35 and press them against your ears really hard, and you get PortaPros. The result is overly boomy bass, just way too much for me. People have succeeded in bending the headband creatively to remedy this, I've never tried it.
Sennheiser PX200: Pretty accurate mids and bass, but treble extension is rather poor, resulting in a rather closed-in sound. Upper registers on violin were particularly affected by this. Pretty good isolation, enough for most purposes. If you need cheap, good portable headphones that isolate, this would be the ticket. Decent soundstage for the price.
Sony MDR-V6: My first good headphones, sentimental value.
Incredible bass response, often too much but hell, it's fun sometimes. Midrange is disturbingly absent and sterile. Highs vary from bright to unbearably harsh depending on the source/amp and recording. Good isolation.
Sony D66 Eggos: Bass is quite lacking, midrange/highs nice, except for a strange "glossy" smooth colouration that I just couldn't get used to. They look ugly in my opinion. I can't remember how they isolate...
Sennheiser HD497: I'm going to be honest, I didn't do much listening to these, so I won't comment.
Grado SR-80: Wanted to give Grado a chance, so I tried these for a couple weeks... I dunno, Grados (at least the current generation) aren't for me. Too bright. Mind you, I only tried them with bowls. But even then, I didn't even remotely like them, so that's that.
Etymotic ER-4S: Accurate and detailed as hell, a bit on the lean and "dry" side for me. Somewhat cold sounding, but it can really bring out details that you never knew existed. The soundstage it presents is truly unique, takes some getting used-to. A bit thin sounding without an amp, but it's still tolerable out of a decent PCDP heapdhone jack. For trains and planes, for me there's no alternative-- absolutely amazing isolation. For home listening though I get a bit claustrophobic with these after a while... so...
Sennheiser HD600 with Equinox: My current reference. I know they're not perfect, but nonetheless the closest to real live sound I've ever heard in headphones. Great sense of depth of soundstage, accurate imaging and there's nothing in particular tonally that bothers me. Doesn't bombard you with details, just shows you what's important... the music. No real desire to upgrade headphones at the moment-- gotta work on upstream components first.
These cans really improve with every component upgrade; they won't be the limiting factor in my system for a good while.