Ok, two days since I got them and I'm at a loss for words. I'm not one of these guys who can spew all the terms like some of you (cold, warm, euphonic, etc) but these TH900's sound better than stock. The bass was always present and strong - it is now so tight and deep and seems limitless how strong it can be with not a speck of distortion or muddiness. And I've pushed them to the "cup shaking level". Rarely do I go that way but I had to test them, right?
Two test tracks I always use on checking out new phones, auditions, recently modded units - Steely Dan's Aja (title track - has an amazing small group of drum solos) and Hurry Hurry and Deeper, both by Pete Belasco off the Deeper album (the whole album is A+). I put them on my iPod Touch feeding through a NuForce iDo DAC into my Pioneer Spec 1 & 2 vintage speaker amp system. Oodles of power (the amp was tested at over 310 watts per channel before distorting) through the headphone out. Black background - no noise whatsoever.
Lest anyone say "low quality files" - I did the same test with the Aja CD and album on vinyl from my original purchase back in the 70's and same result. Astounding sound.
Someone should check out those tracks - good music and great for basshead folks. Anyway, the sound overall is very rich, very balanced, yet is bass-heavy if that makes any sense. What I mean is I don't sense any area that is not clear and well represented. Highs are sparkly (there's one of those terms), the mids sound good (voices are like Donald Fagen is in the room with you) and the bass feels bottomless (no end in sight).
I used to feel they were about the equal of my D7000 (which are Lawton modded) but now they are better. I have to listen to more A/B stuff like the above, which I had no time for last night, before I can say how much better they are, but I cannot imagine a better headphone. Oh, and the soundstage is as close to open as you can imagine. You know when you hear something way out to the back left that you think is someone trying to get your attention? Yep, these do that a lot.
The $129 cost including shipping is a pittance in the world of $400 cable swaps and $300 cup changes. There is no mod I'd recommend higher than Mark Lawton damping - and he has blown me away twice now. First time was with my stock D7000 - sent it to him for the damping and his angle pads - what I got back was a great headphone after sending him a good one. Now I sent him a great headphone and got back what to me seems the perfect sound signature for my liking. No, I haven't heard a Stax 009 but I'm so impressed by the TH900 that I am quite content.
I'm a Denon/Fostex fanboy and after auditioning or owning over 40 headphones I've found the signature sweet spot for me. Own a LA2000 that's got extra deep cups and a D600 as well. Love them all.