When properly amped, I would indeed call the 2900 a bass-head open-back. But it's more than that. It's like high quality speakers. Something in the treble makes it special, it reaches high and far, and feels very energetic. It is certainly on the sharp side with the treble, so if you were sensitive, I'd be careful. It's very bright! Pro 900 doesn't sound as bright to me, but that's probably because of its HUGE bass.
I feel like I'm in the recording studio hearing all kinds of details I've never heard before in any headphone. I started off with the PRO 900, but ever since I got the 2900, I find myself reaching for it more. It's my new favorite hp.
It's an outstanding headphone, so incredibly detailed and clear through the whole frequency spectrum.
If you could only have one, and provided you can properly amp it, I'd pick 2900. You could always EQ up the bass a bit if you prefer as well.
For now, I feel like the Pro 900 overwhelms with the bass. The 2900 pulls back in bass (quite a few notches) so you can hear the beauty of the whole song, not primarily booming bass. The bass of the 2900 is still excellent, just smaller (and what headphone isn't if comparing it to the 900?!). The open back also gives it a more airy transparent sound, which makes for an excellent soundstage.
If you "hate" a recessed midrange, I think you should certainly go for the 2900. That's easy. It pushes vocals out further in front than the 900. For me, I love having them both.
totally agree with you! the pro 2900 are one of the most underrated hp ever made. They sound so freakin good but dont get a lot of reviews. It has been my hp for the past 3 years. I have heard many high end cans like t1,lcd2,he600,he500,hd800,etc and i still like the pro 2900's signature better. pair it with a good amp and hear the amazing quality!