Yamaha CS80

>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).

Feed the main output back into the External Input!
With carefully set modulation level, you can bring in controlled "roughness" 
to the sound
via aftertouch. (At least it works like that on the CS-50.)

JH.