PC > X10 wireless transmitter with L/R RCA jacks > X10 wireless receiver with 10W amp and spring speaker terminals > speakers.

Aha. And I'm guessing that X10 amp doesn't have any bass and treble controls.

Okay, get a relatively-cheap line-level equalizer and insert it into the line-level signal chain before the X10 transmitter.

Also, the subwoofer suggestion was a good one.