The best sounding GM box sets were by Roland - the 55,88 & the daddy the ISA SCC-1.. in fact any of the devices bearing the 'sound canvas' logo adhere to all the basics for a GM device - that is all the instruments and drum sets are standardised and good quality. The issue is connecting them.. The boxed devices all needed horrible mini-din to midi leads, or something equally horrible - they were not bought by gamers at the time, but by musicians after a multi-timbral rompler, and the SCC-1 is an ISA card with mini-din breakouts at the back - but all of them would emulate and pass-thru MPU401 midi, which was nice.
The external 55 & 88 range were just more advanced versions of the MT32, and some have nice LCD displays and controls, while the '32 was dumb, and the sound set was a little bit lame. All of the GM standard included reverb and chorus, and each instrument/channel has it's own aux send for each effect, level controllable by a Midi CC, as was PAN and ch vol. All these were instant, and had no 'zipper' artifacts, so in code you could shimmer from left to right at a ferocious rate, and could pick out individual notes and drum hits, and plunge them into reverb or chorus, while panning, and taking advantage of any extra modulation available in the patch - some had aftertouch, mod wheel etc. available to change the sound, all the sounds were velocity sensitive, some just for dynamics, some (like the piano patches) would x-fade samples. You could also switch sounds without 'clicking' during a pattern, so You were able to build massive soundscapes using all the voices available, but with endless combinations of tones per instant. 'midi code' could make use of a resolution of 192ppq - that's a shed load of events to play with, and although the bandwidth of midi is pretty poor by today's standards, it was very hard to run out of slots - tracker this was not!
GM became a standard on many instruments, just as a way of laying out patches - so Pianos first, Drums default to channel 10 etc. and most workstation devices in the late 80's and the 90's had at least one GM bank on board - all of these instruments could be used to score a DOS game, if You selected the GM bank on the channels used in the game. You do NOT need a sound canvass or any GM compatible box, all you need is a midi port in the machine, as all the GM devices in the dos days pretended to me MPU401 interfaces. Just use the midi out and send it to Your choice of machine! For the 'Deluxe experience' pick up an EMU Proteus, it was one of the last multi-timbral monsters that could spoof a GM set, and it's waveforms are top of the range sexy, and they are cheap now. Forget low-res samples and fizzy drum hits, use an orchestra!!
Unfortunately maker of those said that because of this global parts shortage, making those cards is impossible right now... 
