I've finally stopped using the onboard audio and gone for a Sonica Theatre and two stereo amplifiers. The M-Audio software is configured for a 4.0 setup.
Both amps work fine, but the only way I can get the M-Audio card to output anything out of the rears is to enable their "TruSurround XT" rubbish at which point only the effects are sent to the rears.
I just want the rears to be an exact copy of the fronts, is this possible?
Also, Centrafuse doesn't see the rear channels, so does not enable it's rear speaker sliders. I suspect this may be related.