Got two upgrades in:
An Edifier R1100 speaker set to replace my vintage, plastic, yellowed speakers. The R1100 has a reasonable price (~70 euros), at least for what I like to spend on retro gaming. The quality improvement is huge. Of course SC-55 music sounds much better, but also things like the Doom chaingun and FM music in general sound much less tinny/annoying. Also these speakers have an aux in that's mixed into the standard signal, which is great for MIDI devices.
The other upgrade was a Noctua 80MM fan for my machine's power supply. A huge pain in the ass to install as the complete PSU had to come apart, although the fan did plug right in without any soldering. The difference in noise level is huge, my retro PC used to be noisier than my main PC, now that's no longer the case.