If you are OK with d-class amps a cheap way would be to use car amps and DC power supplies, have bolted to the bottom of desks to get them out of the way and get some sound going. You will need 2 of them if you want quadraphonic output on your pc (one connected to the main LR and the other connected to the rear LR).
Alternatively you can get some of the cheap standalone amps like the
Micca OriGain w/ digital input or bump up to something like the
Audioengine N22 which will give you Class AB (but no digital input, for best quality a decent DAC to pair would be something like the Dragonfly Black, a decent 32bit kit but will add another $100 unfortunately).
The bottom line is just how much you want to spend to get quality output.