Why bother goin from DPIF to analog? Just buy a digital amp.
Well digital amp first you're stuck with one huge amp. Usually the sub output won't be enough for the taste of many (like 100 Watts). Plus you get way more flexibility qith mor ethan one amp (that you can use for each set of speakers for perfect managment (time delay, etc).
And also, I'm pretty sure that the separate converters are way more powerful that the inboard one (more bits, control the time delay from there, etc)
Anywyas, a digital amp should be fine, it depends what you want to do and the control you want.
if you want to go SPDIF, you shoul go all the way with it, and end it right before the amp, what motherboard and what Soundcard are you using, I have been researching this topic quite extensively in the past few days, I have decided to go with the Audigy 2 NX USB Soundcard....3.5mm to RCA Stellar Labs Interconnects to a AudioControl Matrix 6-Channel Line Driver then to the amps, im hopeing the signal will be clean and the output voltage will be around 5volts which is better than the soundcards 2volts.