Well I have some things to report.
First, I ended up getting a pair of the ATH-AD900X, in fact they just arrived and I'm giving them a listen. Initially I couldn't tell that big of a difference between these and the AD700's. The bass was obviously more pronounced, but it took a few songs to compare and now I am starting to realize. One way to explain the sound is the 900's seem cleaner, if that makes sense. The highs and mids don't sound as artificial. That's it! I think the main difference is the music simply sounds more realistic. Each instrument, and each different sound has it's own character.
Well needless to say I really enjoy these and am glad I invested the extra bit. Now the next step is figuring out the input set up.
After considering all of the suggestions, I am considering the following possibilities:
1) Invest in a Asus essence STX, use it to power both speakers (Klipsch promedia 2.1 for now) and headphones (AD900X).
a) Also get a FiiO E09K: STX -> E09K -> headphones; STX -> speakers
2) Use onboard sound card for gaming (it works fine for me) and get a nice DAC/headphone AMP for music.
The STX and Fiio E09K use the same TI 6120 headphone amplifier chip.
Give the AD900X some break in time, you will like them even more.
As the AD900Xs are only 40-Ohm, you would want a headphone amplifier with a lower impedance then the STX's 10-Ohm.
Schiit Magni external headphone amplifier, $99, has an impedance of less then 1-Ohm :) and as it is external, can be used with more then the computer.
Asus Xonar DX or D1 sound card, sells used for $50-$60, same audio processor (CMI8788) as the Essence STX.
With the Xonar DX/D1, you can then plug the Schiit Magni into the computer's front panel headphone jack and leave the speakers plugged into the Xonar's back panel.
Still think once the AD900X have broken in, you will stop using the Klipsch anyway.