Any Squeezeslave + SB player + audio card combo can drive an external amplifier and yes you can use the line output. level to control the audio volume to that Amp. If you have several zones/rooms with that kind of setup then you can use a separate IP/rs232 controlled audio matrix ahead of the amplifiers (ie audio card outs go to the inputs of the matrix...outpts of the matrix go to the amps) to do thinks like route one zones audio to any or all outputs & therefore deliver perfect in sync audio to all of those zones/rooms.
An alternative approach which delivers the same capabilities but in a more integrated & compact solution is to use a matrix amplifier such as the RTI AZ-4 or AZ-8. These are 4x zone & 8x zone stereo amplifiers with an integrated audio matrix that allows any input to be routed to any or all outputs (you also get control of mute on/off, vol up,down and equalisation & balance per stereo output etc). The advantage of these units is that they reduce cabling in the rack and are simpler to control as they are integrated units. The downside of these units is that they are pro-install products so they are not bargain priced - however if you need 4 or 8 zones of audio once you add up the cost of 8x stereo power Amps, 8x8 audio matrix etc etc... the RTI's are actually pretty competitive cost wise. The AD-8 I think is about $1650 USD so thats about $200 USD per zone for a matrix and power amp.
Most of our pro-installers are now going for this type of audio installation using the RTI's or SystemLine AZ-8's (which do much the same but use in-ceiling powered speakers with integrated Amps)
All the best