Hmm, sound like a tough job...
If you're going for the best possible signal quality, I'd recommend recording the OPL2/3 output digitally. This is possible with the Soundblaster AWE line of cards, which feature an onboard SP/DIF output connector (it is only advertised with the SB AWE64 Gold, but most AWE models have it, you only need to build a very simple connector cable).
The good thing of this method is that the signal doesn't get converted to analog in any stage of the chain (OPL3 -> EMU8k -> SP/DIF -> recording device), and the usually noisy SB Mixer doesn't do any harm.
The bad news is that you need a digital recording device plus an AWE card, and you have to be rather picky about the AWE model, because many of them didn't sport a real OPL chip any more, but came with Creative's CQM FM emulation, which sounds a bit different.
For analogue recording, find a soundcard with a real OPL3 and quiet mixer. Creative's cards are a rather bad choice, because the ones with a real OPL chip are very noisy, and those with better sound quality usually come with the "fake" FM (as mentioned above).
Some cards from other manufacturers are reported to have a better output signal, however I can't say anything about these (Maybe other VOGONS members know more about suitable models).
If you really have to resort to analogue recordings from a Creative card, take a look at the Master Mixer utility, which I reviewed here.
(part of this epic thread over at QuestStudios, in case you're interested in some SB16/AWE details).