DAC vs. Preamp for 5.1 System?

TAS reader Bob P. asks:
 
What are pros/cons of using a dac alone  to power amps for digital sources in a combined two channel/HT rig; vs using also a tubed preamp? Sources that need to be integrated include a transport; a music server; subwoofer, directv and Blue ray video sources, and zone two. Main system is 5.1 with pair of monoblock mains; and separate 5 channel amp; or with what my needs are; must I have a preamp in the chain with HT bypass for unity gain; and control of volume by the video processor…Thanks for considering my question…
Enjoy the music,