ssddx :
no. just no.
first, the ad900x is fairly easy to drive. with 38ohm impedence and 100db sensitivity even most portables can power it. generally any soundcard, onboard sound or similar can as well. pairing it up with two powerful amps is not going to help.
the creative Z has a very powerful amplifier and with headphones that are easy to drive you're going to be running the volume control at low settings (otherwise the volume level on the headphones will be loud enough to damage your ears). that without putting an auxilliary amp on them like the fiio.
the fiio e10k is also a pretty powerful headphone amplifier and if you used this (without the soundcard on just an aux-out jack) you would be running it below half gain.
if you have both connected using both amplifiers, you would need both at minimum settings in order to even listen to it and you would be getting more distortion due to amplification than you really need. a big waste of money as well. the creative z doesnt have an aux-output or line-output which makes it a poor choice for external amps unless you're trying to power some really hefty headphones like the dt990-250 or 600's
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back to square 1....
first... are the ad900x what you really want in a pair of headphones? while they have absolutely superb soundstaging and positional audio as well as crisp clean sound they are also incredibly bass light. while this gives them draw as good gaming headphones for fps (gives you an advantage in this case) they also arent very "fun" headphones to listen to because of this unless you really enjoy that type of sound signature.
second... why did you want both a soundcard with powerful amp AND an external amp? why not just a soundcard? even the xonar dx which doesnt have a strong amplifier should be able to handle them fine and the dx uses the same DAC as the top-of-the-line STX (but doesnt have the amp and a few other things). the dx also pairs better with external amps (shouldnt be needed for the 900x) provided you went with one since it doesnt have as strong of an amplifier than cards like the z. a line-out would be best though.
third... what are you expecting out of the system (why all the amplifiers and high power soundcards?). if you liked creative better than asus you could still use the Z (at low volume levels in windows) with the ad900x but you definitely do not want to put an external amp in the loop as well.
Thanks for the informative reply!
Now I've understand what is not necessary and what is necessary.
AD900x is affordable headset and based on reviews its one of the best for gaming, but as you said it lacks the bass
Basically,what I want is a setup that is very good for competitive gaming (Virtual surround sound) + watching movies (Bass?).
My budget is 300$ and currently I'm using asus rampage v extreme on-board sound (SupremeFX) + Beats solo
Can you please suggest some good headsets/headphones, sound cards... etc ?
As i mentioned before, the setup will be for gaming and watching movies.
Please note that what you will suggest is what I will actually buy!
Thank again for you time!