Yamaha CS80

Does anyone use this?  What's the typical way of utilizing this 
feature?

I'm used to synths with an External Input to process another 
instrument through the filter block, but this seems to route the 
signal only as an aftertouch modulation source waveform.

Is there any sub-audio source that's good to use?  Most signals I 
could think to apply are audio frequency, which would seem to give a 
noise or ring-mod like character to the modulation (maybe anyway, 
haven't tried it yet).

Finally, is there sensitivity of the behavior to input level?  As far 
as the audio frequency sources I can think of, a microphone has a 
totally different level than a synth or other line level source.

Of course, I could just play around with it, but thought I'd ask 
before dragging it back from the wall and trying it out.