Don't need an amp or anything exotic, just plenty of outputs. I know this has been covered in a lot of threads but I'm going to happen to be at the local computer outlet today so if I happen to get a suggestion...
MOTU Ultralite has 14 outs in total. $550 though.
Perfect for your application though. Audio Express is cheaper if you can manage with 6 outs.
M-Audio used to make the least expensive model I know of with 8 outs.
Don't know how easy it is to find now.
Look for a Fast Track ULtra.
Hmm... are you planning to output from one JRiver player or from many JRiver players to different devices through S/PDIF ... ?
I first answered your question on the assumption you wanted to send different program material to different zones.
If all you want to do is send the same signal to multiple locations it might be cheaper to use a distribution amp. That's what they are for.