After discussing the issues that I’ve been having with the ASUS Xonar DG causing general system instability, I have now heard from various sources that it could be the “GX” feature of the card causing the problem.
The GX mode is ASUS’ attempt to emulate EAX (Creative’s surround sound engine) so that older games written using it can still have surround sound. What they don’t seem to tell you is that it might cause all kinds of system lock-ups depending what games you play, etc.
As such, I feel that I perhaps should give the Xonar another go, this time with the GX mode permanently set to off. If this cures the stability issues, then I will happily recommend the cards again, with the proviso that GX mode should be always off. Especially considering that almost all games released these days are not coded with EAX and so gain no benefit from GX mode.