Just remember to configure your Xbox digital audio output settings to be plain stereo instead of surround sound.
A valid point, but since we only needed it to work for the duration of a kids' birthday party, a little late

They were only playing Lego Harry Potter, etc., so I don't think it particularly mattered...
It turns out that the weak link in my PC audio chain is these. Terrible mains hum when using the headphone socket on the front. I'm going to wander down to Richer Sounds in Chiswick, probably tomorrow or Thursday, and have a chat with the nice man about my options.