Alright, so these are powered speakers, sounds like what you need is a prepro, sometimes called a surround preamp. And it needs to take (optical) digital in. You mention 1/8's, but don't those connect to the sub? Usually the multichannel amp is built into the sub in those 5.1 systems. So what sort of inputs does the sub have, also 1/8's?
Any chance you can just sell them and buy the Logitech Z-5500 Digital (or equivalent)? Then you wouldn't need the prepro at all.
Alternatively, your other best bet would be to haunt garage sales, yard sales, and eBay and look for an ancient pre/pro, or 5.1 receiver with pre-outs. Someone on AudioKarma mentioned the old Denon receivers, I think.