Ive tested couple of ASUS soundcards, ST amongs them (its same as STX, only its PCI not PCIe .. which in fact is tiny bit better .. STX is PCIe converted ST).
I had Forte, now I have Bravura. ST is quite good, but sounds bit on tin can side (probably due OPAMPs, which I dont know if will affect sound via RCA-to-XLR). Forte sounds really good, but Bravura is way better than this. Tough, on other side, my old Audigy 2 ZS with kX drivers sounds great too.
In fact better and more pleasant to my ears than Forte or ST/X.
Most of soundcards have DAC, but quality is not comparable to regular DAC. They are sometimes close, but if you compare it to some decent DAC, its not that good. Using soundcard as source for amp isnt bad idea and for lots of ppl its very affordable idea, but separated DAC is bit better. You wont believe until you hear I guess.
AKG 701 are very demanding headphones (as much demanding as your ears are) and definetly not forgiving. They can sound really great, if you hit right combo.. or downright tragic, if you dont. Thats why quite a lot of ppl doesnt like them. On opposite site, those who have that "right combo" or just bad hearing, likes them.
Definetly not begginers headphones as I learned fast..
PS: I believe that theres lot of ppl that believe in "brands". Well, ASUS does lots of things.. in fact almost everything you can imagine. But that really doesnt mean that they do that good. Usually, they just know how to sell it, nothing else. Thats why I didnt post any review about those soundcards I tested. Cause ASUS wouldnt be exactly pleased with results and that would probably result in ASUS not liking that certain site.. which would mean no more good things from ASUS for that site .. which lead to editor saying "No." Thats how our world works.