PROPHET64 & MSSIAH Userforum

The 5th pot is probably in most cases the popular feedbackloop, adding more crunchy resonance response up to drastic distortion.
I dunno about the 6th, but I was hooking up a passive attenuator as a main volume control to one of my boxes.

The switches usually are there to bypass the potentiometers, since they freak out a 1351 mouse when used with the sequencer application; it's possible to use a single DPST or DPDT switch instead.

Another use is to ground the SID's Audio-input(s) with a switch, due to one's personal preference (instead of NC-jacks or dummy-plugs).

Again, it's really instructive to read AlphA's "mother of all mods" http://www.bigmech.com/misc/c64mods/
It covers pretty much everything.

About helpful mods? It's just your personal taste, what you think to be suitable to reach your individual sound
Honestly, If you're using mostly the sequencer I don't see a huge demand to install internal POTs and bypass-switches or the feedback mod. If you, like me, prefer the grooveboxes (Monosynth, Bassline, Drummer) you probably will be desperate for those.

There are a couple of threads about the best way to do install the potentiometers, It seems like the connection diagram from the Prophet64/MSSIAH getting started manual isn't the best way;  see the Making Pots More Accurate thread and go for firestARTer's schematics (100K pot & 4.7nF cap) instead.

Your question about "best pots" may lead to as many answers as there're different pots available, my favorites are 16mm ALPHA potentiometers with round shaft.

Whatever, just avoid cheap pots (like radioohm) with poor housings and/ore plastic parts.

Certainly useful as well;)