I'd look at Lepai LP-838 and LP-168HA amplifiers for doing this. speaker power left right into a line level adapter, then into the amp inputs. you get left-right out with volume bass treble, and a subwoofer channel volume thats already low pass filtered so its very simplified.
the LP-838 about 13 bucks ebay or amazon, good for roughly up to 7+7+13w.
the 168HA about double that (real world not their cheezy specs) and Spooky uses them in AMH.

either amp probably more usable power than what comes in most pinball machines and the machines onboard audio is basically rendered into being a cool running preamp.