Regarding the OPL chips, I'm quite surprised that most games that support the OPL chips on DOS didn't sound as good as the YM2612. It was clearly possible to get some extremely decent music out of it, but composers didn't bother as most companies (eg, Sierra and Ocean) used the Roland MT-32/LAPC-I as their main supported hardware. However, most of the sci-fi games such as Tyrian and Descent really made proper use of this chip.
Here's some samples of the OPL series sounding at their best:
Tyrian
The Level
Torm - The Gathering
Asteroid Dance - Part 1
Descent
Level 1
Level 11
Level 22
Descent 2
Menu
Level 2
Level 3
HOWEVER, there's one composer who made the MT-32 AND OPL sound good - I'm sure we've all heard of him - it's Rob Hubbard
Ski or Die for example:
Downhill Blitz (OPL2)
Downhill Blitz (MT-32)
Too bad not all composers were like Rob
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