Heya,
Nice review! I like that you maintained that it's not all about the bass, but sometimes, the kind of bass in relation to everything else. For me, as much as I loved the bass of the PRO900 it just made it too difficult to listen to a lot of kinds of music that didn't need that bass, and it particuarily was disturbing when trying to enjoy some music that had vocals in it, as the vocals just sounded really metalic and damp at the same time. Also, an acoustic session shouldn't sound like a bass drum heh. I ended up gravitating towards the Denon D2000 simply because I realized what kind of bass I was after, and that's basically sub bass. The PRO900 and D2000 can both hit nearly the same bass, but the difference is the volume in relation to each other. The PRO900 has a lot more mid-bass hump that is pronounced, so mid bass sounds louder than sub bass, which gives you the perception that it doesn't do sub bass as well. The D2000 on the other hand is flat, so you hear the same volume in sub bass as you do in mid and upper bass. I find that to be a better bass for me, even though I love big bass, I like being able to have sub bass drops that don't drown out the mids, a flat headphone can achieve that, and since it can nail 30hz with authority, yet you can still hear a feather drop, you realize what kind of sound it is. I consider that to be more of a high-end sound. The PRO900 is a super bass can, but once you've heard another headphone that can also do that much bass, yet still provide mids and a nice sound for all things, you realize what you're really after. For me, the HE-500 was actually the reason that I was able to let the PRO900 go. Then I got the D2000 and found the "middle ground" so to speak, between mid-tier and high-end that did the high-end sound and let you enjoy serious bass while still having a headphone that can cover all genres. Moving up the ladder, the D5000 is definitely the way to go in that sense, it's a bassier, warmer D2000, which is just yummy goodness.
One last plug, definitely try the Panasonic RP-HTF600's. It's unreal what those headphones do for the cost.
Very best,