k501 is very overrated. Not sure who started the hype machine on those. It usually will take a year to find one if you don't use Ebay/Paypal. I finally found one and got rid of it once I got the Q701 and had the K601.
If you want a taste of the K501 sound, the K400 is quite similar. I have two of them now and with my Q701 around they don't get much use. I still like them though. They sound like a poor man's Q701 and lack low bass a LOT. The low bass is worse than even the K501. They're great for female vocals though.
I've seen graphs of the K612 on Goldenears and they do look like they have less treble peaks. Strangely enough the K601 itself was close to being dark overall already! Not sure if i'd like the K612. It's cheap so I may get one.
K601 was one of my favorite headphones and I used it over the K702, but not the Q701. I actually had the K601 and Q701 together for several years.
Oh and K501 definitely has the larger soundstage out of any headphone i've ever heard. It's almost too large! Playing Fallout 3 with that on your head is quite the experience. Go into Tenpenny Tower with that and you feel as if you're right there and everything is outside your head.
BTW to me the K601 always sounded like a mix of the K702 and HD-600. K601 was always warmer than the K701 but not the Q701. K601 felt better for rock music and metal than the Q701 for whatever reason. OK, I didn't really listen to metal unless you can call Primus that. Primus always sounded good on the K601!
There's been a K501 on Amazon for about 2 years for $999. You'd have to be stupid to buy that. It's not even worth $300. You'd have to probably pay $200-$300 to acquire one unless you're lucky and constantly check Ebay.
Oh and the K400 is usually going for $100. I got mine last year on Amazon for $65! Came in a stinky old box from probably 20 years ago (or seemed like it!). The headphone was mint but smelled a little dusty.
Edited by tdockweiler - 6/25/14 at 10:34pm