My initial reflex is to suggest a beat-up old receiver or integrated amp from a yard sale, thrift store, or junk pile. But your garage may actually have a floor... that's painted? Perhaps a now-obsolete non-HDMI Dolby Digital/DTS AV receiver. That makes it easy to add a sub since it has bass management and the .1 output. You can upgrade to surround sound later with more speakers. Some receivers allow using unused speaker channels for a second zone, or doing a form of biamping which would give you two lefts and two rights, to feed two sets of speakers. Shouldn't cost more than $100.