Holy frick, so...this hint I'm getting of using FM to create acid...is totally achievable on the xoxbox alone?
Gotta look into that for future hardware.
But the thing that is really "acidic" is if you modulate the modulation freq and depth like I did with the FDS instruments. I think I only hear you messing with cuttoff and resonance. Can you live twiddle knobs for the FM processing itself?
Not that I know of, but on a similar note, you can create your own hardware to do this; a band-pass filter, a phase-locked loop & a low slew-rate op-amp audio amplifier is all you need to really mess around with waveforms.
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So... I've just learned that the FDS's modulation table can run up to around 1700Hz or so (all the pitches that the main wavetable can play at). This thing was totally capable of FM synthesis, though we can't really do it in FamiTracker without one instrument per pitch used (the pitch table value should be your modulation rate). Also there's no effect/macro for modulation depth so you'd be stuck with a static wave and might as well just use the wavetable for that directly.
If there were an option to lock that modulation rate to the wavetable's frequency (or integer multiples), and to envelope the modulation depth, we could reasonably do FM synthesis with the FDS.
Though... did any games do this?
Edit: this thing was not totally capable of FM synthesis, see below.
Ha ha, after experimenting with it I'm beginning to realize why I can't find any games doing this. There are two big factors against it:
1. Changing the modulation depth creates a big pop, which more or less kills any desire to envelope it.
2. The resolution of the frequencies available doesn't really allow the carrier/modulator oscillators to stay properly synchronized, the carrier will shift phase a little every cycle creating horrible phasing problems (I think this is also why a lot of FM synthesizers implement as phase modulation rather than true frequency modulation).
So... never mind. It actually seems pretty much useless for FM. Though, I'm sure everyone in Japan already knew this 25 years ago.
Yeah, after doing this experiment, I don't think FDS specific effects or the ability to match pitch with mod rate would be worthwhile anyway.
I have listened to a bunch of FDS games on that list. The best sounding one so far was the FDS version of Gyruss.
The only one I've seen firsthand using the modulator at frequencies above an LFO level was Nazo no Murasamejou, but it was doing it on a really weird waveform anyway, it kind of seemed accidental, didn't even affect the sound much the way it was done.
) 
Though, I'm sure everyone in Japan already knew this 25 years ago.