Solutions so far for surround output as well as stereo from a PC:
DAC (Soundcard + possible additional Amp depending on the card) inside the PC.
DAC (Soundcard) in PC with Amp (O2 or Magni) outside.
DAC in PC (any soundcard with a SPDIF output) feeding surround signal to an external DAC/AMP (with SPDIF input: Audio_GD say) or separate DAC (again with SPDIF input) and an Amp. In SPDIF mode, the surround signal from the soundcard is preserved.
From what I read, the PC itself provides the power source at least; also the CPU does some processing too in surround sound if you're not going the SPDIF route; so perhaps we should factor in it's presence in the equation/chain?
Again.. amazing replies from you guys. I agree the further they are from potential electrical interference in a PC case makes perfect sense (mine's is overclocked but not heavily, plus I have a reasonably high end GPU) Thing is.. the above options are really just shifting the same components around from inside, half out the door to almost completely outside of the computer :)
Would prefer to at least have an external amp. OK .. how about a STX/ZxR with a low output impedance O2 or Magni amp to pair up with say the MA900 or X1 for a bit of double amping? Pre-amp to amp.. been done before? Even my old ultra low impedance FiiO E5 stuck in the back of either as a starter (via the headphone out) might well smooth out any potential distortion due to the 1/8th rule.
Am however open to all and any suggestions. Happily I'm old enough to put plenty of time into researching this stuff. Unhappily I live far away from any dealers that let you hear their headphones - I've only heard the X1's out of my mate's IPhone (he's a sound engineer - but rarely ever in Britain).