[MrSpaceFox]

Both will be within my budget, but I only have a fiio e06 driver and don't plan on getting another. Are either pair outdated or less reputable? Thanks!

 

[Za Warudo]

Don't bother with the Q701 unless you have a desktop amp or a decent portable amp, most Fiio amps won't be able to drive them properly. Don't know about the AD900x.

 

[PurpleAngel]

  Both will be within my budget, but I only have a Fiio E06 driver and don't plan on getting another. Are either pair outdated or less reputable? Thanks!

 
It will be much easier for the E6 to drive the ATH-AD900Xs, then the Q701s.
If the E6 can even drive the Q701, chances are the E6's volume would be maxed out and not bringing out the Q701's best.

 

[Lumos]

Every headphone has it flaw including these two. It is you to decide which one fit your needs. I do have Fiio E06 and Q701, (but I have not listened them together I believe) so if you wish I can do some comparisons how they together perform.
 
both will be bass light, compared to real world IMO, both will be somehow bright sounding... as far as I can tell Q701 is very good headphone with nice tight bass, good speed/fast decay. cons are: spike somewhere 2-3K and uncomfortable headband if you have larger head...
 
describe more what kind of sound do you like, what you are listening and better options might available.
 
cheers

 

[LajostheHun]

Like others said, with low power amps the AT is a better choice. Of course I would get the ATH M50x over the 900x, but that's just me. I own all 3 BTW

 

[DJDREEM]

Get the Q701. Not hard to drive at all, out of a simple E6 it does fine but gets better with better gear.