The output of your sound card is 3.5 mm. You need 3.5mm to RCA male adapters, to plug into the existing amplifiers. The amplifiers don't decode the surround, the computer is decoding it.
There is no advantage to using one amp, you already have what you need. Unless you really "want" one amp.
In that case ONE amp, with 4 RCA inputs, you just need the adapters.
You will have a hard time finding an amp with 3.5 mm inputs. Almost always, you will need to use adapter / patch cables, 3.5 mm stereo to left / right RCA male plugs.
There is no advantage to having the amp decode the signal, and a cheap amp is a downgrade. The computer decodes the surround and the analog out goes directly to an amplifier.