***SOLUTION!!!***
Well guys, I Googled around for a while and found a thread on an Antec support forum, on which people were saying they solved their front panel buzzing by disconnecting the front panel earthing screw.
The screw was causing a ground loop, because the front panel sockets also have their own ground in the multi-cable!
This got me wondering if I had a similar problem as I had all the same symptoms described ...I couldn't see a screw to the chassis, but then I noticed this:
The front plate is in direct contact with the chassis, so that's where I reckon it was earthing. My case is a Lian Li PC-7 Plus.
Removing the panel from contact with the case resulted in zero detectable background noise through the headphones, yay!
So I wrapped the screws in PTFE tape and put some electrical tape behind the front panel / chassis contact points:
Ideally I'd make sure of a lack of electrical contact between the screws and chassis, by gluing the panel on or something, though unless I'm extremely precise, the front panel won't go back on.
I then set about fiddling with the settings.
I actually found the following very good for music through headphones (Sennheiser HD555):
Audio channel: 2 Channels.
Sample rate: PCM 96KHz.
Analogue out: FP Headphones.
Dolby headphone ticked, setting: DH-1.
Amp mode: exciter (the highest gain).
FP mode: AC97 (as opposed to HD, I gather this is determined by the type of connector on your front panel).
30Hz and 60Hz tweaked upwards a couple of notches on the crossover.
Flexbass and everything else off (including GX).
Front switching still seems flaky (seems to not switch if the program isn't actually loaded fully, rather than TSR'd).
Conclusion: I'm definitely keeping this card now, as I'm actually picking up some details I wasn't before. I still don't think the SQ is quite there in music, and the drivers seem very poor.
However, the convenience and the fact that the SQ seems better in terms of pure details (hearing background voices / noises I never noticed before on certain tracks), along with the fact that the buzzing was (as was always likely) nothing to do with the sound card, means that this card is now very much a keeper.