i'm a bit tired tonight and not in the mood for writing a thesis but here's my opnion after having owned Denon's, Lawtons, AT woodies and also the HD650.
Every post after (and including) TWerk's sincere review makes damn good sence and is valuable info to people into or considering these kinds of cans. TWerk just made this thread really interesting!
This doesn't mean people who can spare the money shouldn't buy the TH900 though, if your budget at this point in life allows it and you are far enough into the Head-Fi journey they are a really good buy. But DO take this as a journey and do it one step at at a time, that's where the fun is at and what will bring you a lot of joy.
One last remark about the HD650. I agree with the opinions here in this thread on the HD650 but everything about these phones changes with a silver cable. And yes I spent some money on phones cables and a wole lotta more on speaker cables. And no I don't belive in recabling or cables anymore. With the exception of silver for the HD650. And no, I won't get into a cable discussion here as this would hurt the interesting discussion we have going on right now. With cables as with most things, to each his own opinion.
I believe you should sell the TH-900's for now TWerk and try some D/LA/2000/5000 or a nice used pair of TH-600. For now... Perception of money, appreciation, sound quality, music tastes and sound signatures you like (with different genres) will change over time. And then there could be different phones perfect for different genres to your ears. This 'll be a long, long journey, but that's what makes it great!