Linux Getting analog audio out even when in pass-through mode

I have an Acer Revo R3700 with an Intel HDA soundcard. I have headphones connected to the headphone out of the HDA soundcard, and via S/PDIF optical I go to an amplifier capable of DTS/AC3 decoding. If I disable passthrough, when I consume media both the amp and headphone get the output, I can mute one or other using alsa and life is good.

When I enable DTS/AC3 passthrough all analog outputs suddenly get nothing . What I would like to do is keep the analog output (obviously there would be a superfluous decoding going on when using the amp, but I could then switch to the heapdhones at all time). I'd like it if I could do it on the fly with XBMC but this seems to require a stop/start of the movie.

Is there a way of doing this?

Thanks,
Brendan