Unless you get good results through an external amp not a chance - the GSX has basically the same output power as a mobile device.
On paper no reason why you can't pair it up with an external amp but some seem to degrade the surround reproduction and others have reported distortion/clipping but I'm assuming they've just not got the input and output levels set right.
Really? that crap huh? Does it not output enough volume? I've used HD800's with a mixamp and while the sound sucks in music test or overall sound quality the volume and VS is sufficient for me. I'm really just after something that can do 7.1 at a decent volume not really caring about overall quality, purely competitive.