Roland/Edirol, Korg, Kawai, and Yamaha - and probably a few others - made cheaper, "headless" versions of the same thing which were designed for exactly what you want to do. IE, no front panel controls or displays, just a blank box that plugs directly into your Mac's serial port and gives you a GM, XG or GS (or similar) soundset to play with, with all editing in software. They're a step up from the MT-32 mentioned above, but still as cheap as all get out these days, especially because they don't have USB. Most of them will also do MIDI passthrough, so you don't need an extra box to plug in other instruments and/or controllers.
Also, most of them were the same width & depth as classic boxmacs, making them zero footprint sound expanders
