Talking about the quality of the motherboard sound. I don't remember if it was better before I added the adapter, but it's definitely got a harsh sound to it. This is not apparent in the analog output.
The recorded tv sound with the PVR250 is as good as the cable signal.
It hasn't been that much of an issue to me since I use a different PC for music (with an optical connection to my receiver).
The M9000 has SPDIF_SEL jumpers on the motherboard. It's two 2-pin jumpers side-by-side. On one end, the two pins are tied together and connected to the center of the RCA jack on the back of the motherboard. The other side of each goes to the signal sources for S/PDIF and composite video. These signals are always available (if configured in the BIOS) - all the jumper is doing is selecting which one goes to the RCA jack. I have the jumper set for composite video. You can grab the signal pin of the S/PDIF side of the jumper and run that to the center of another RCA jack. This can be mounted to the case or on a cable hanging out the back, if you like. You need to pull a ground from somewhere to go to the outside of the RCA jack. I did not, but I'd consider getting this ground from the RCA jack on the motherboard (it appears to be common for both signals). Just solder a wire to it on the inside of the case.
In my machine, I built a driver to take the coax signal and convert it to optical. It's powered by the floppy drive power cable (I think I put a new plug on it). I forget what it looks like; I'll need to dig up the schematics. I don't remember if I took a picture of it and now it's burried under the PVR250. The power connection may be what's causing the harshness, but I really think it was there using the original coax output.
I should explain some more about the harshness (I just remembered more details) - the digital output sounds very good when I play a DVD or recorded video. It sounds harsh only when I play mp3s (or other music files). My theory is that it's not a hardware issue at all, or at least not at this level. I think it may have something to do with resampling my 44.1 kHz sources to 48 kHz for output. The DVD audio that's at 48 sounds good - the mp3s at 44.1 don't. Whether this is in hardware, drivers, or my receiver I don't know.
The other digital connection to my receiver (different computer playing mp3s from winamp) does not resample to 48 kHz, it sends the 44.1 kHz signal and sounds really good.