I think only the super twin with reverb uses the triple-triodes. The standard super twin WITHOUT reverb uses only common dual triodes in the preamp and pi. It seems the reverb needed two more stages, so I guess using triple triodes allowed them to use the same chassis by converitng two dual triodes into triples. The good news is that you can use a chassis punch to put in another tube socket without making metal chips and convert the others, and change over to all common dual-triodes if necessary. One thing this amp definitely has going for it is that it's old enough to use point to point terminals on a strip instead of a printed circuit, so modification like this are usually straight-forward and nearly trivial if the heaters are in parallel; yet it's new enough to have decent modern capacitors.
If finding the correct switched pot is difficult, you might be able to substitute a push/pull switch pot or drill a hole and mount a toggle switch in desperation (put the switch in the back if necessary). I have, on occasion, managed to transplant a switch from one pot to another, so note the brand of the broken pot and if you can't get one with a swtich get one without and see whether you can transplant the switch or reverse transplant the shaft. Note that the rear footswitch jack is also involved in the circuit. BTW nobody likes that distortion 'feature' anyway. Nobody likes the distortion mode of the ultralinear output section either. So if you need dirty, use the master volume to get some preamp distortion...if necessary add an FET gain stomp box.
The stock super twin and super twin with reverb as well as the closely related sister amp the studio bass amp were all very top-heavy. When the wheel caster bases rotated toward the back the support wheelbase was very unstable with front weight. It is very common to find the super twins fell over on their faces. It would be nice if the cabinet ends protected recessed knobs, but no, the knobs protrude. The super twin I bought had many broken knobs, and that's typical. Even loaded in a truck with the cover on the amp and facing a flat wall the knobs get broken. Perhaps you were just unfortunate to have the pot break before the knob.
The stock super twin combo is a really heavy chassis with huge power transformers. THe chassis is held in by the hanger bolts threaded into spring-clips. Unfortunately the spring clips are on the sear side of the chassis instead of going thru the chassis to the far side. So the heavy super twin chassis tends to bend and twist an pop corner welds too. You might consider longer bolts, drill the other side of the chassis, and install nuts there too.
I don't usually condone changing nice classic amps, but IMHO the best thing you can do with a super twin is to install the chassis into a head cabinet. The super twin combo is just too heavy to move with the big trannys and incredibly heavy square ceramic magnet speakers in one package. If you leave it a combo, remove the wheels and put it on a slightly larger wheeled base instead, so that it doesn't fall over on its face anymore. If you leave it a combo, consider installing some of the excellent lightweight Eminence 12" speakers with modern neodymium magnets, or some good lighter alnico magnet speakers and eliminate more than half the speaker weight. As a seperate head and seperate 4X12 cab you won't ever worry about blowing your speakers. Or breaking your back; worth taking two trips to avoid. And with the right speaker cabinet the super twin chassis also makes one of the best single-output tube bass amps ever created (outside of a few wonderful new Fenders and the SVT). OK it doesn't have channel-switching etc. but it will hold its own against the really big tube bass amps for one third of their price.
If you convert it into a head, it needs a LOT of cooling. The combo has a lot of air inside. The head doesn't. I made my head cabinet a bit taller, and also deeper so air could get out past the chassis, then cut back the top of the cabinet to allow access to the jacks in the back. I also added vents in the bottom of the cabinet, taller rubber feet, bigger oval air inlet cutout in the rear panel, and the littel grille cloth panel in mine isn't solid, it actually passes air. I don't like fan noise, but consider a really quiet one.
If it's a super twin without reverb, there's a guy on ebay now with the very last OEM NOS super twin faceplate in known existence. I bought the next-to-last one, and they clean up to look perfect.
The EQ knobs are numbered with 0 in the middle so they are not standard Fender knobs, and they break. So if you leave it a combo see it you can pick up a few spares from Fender or on ebay before they disappear. Of course, you can always change to chicken head knobs for the EQ and get an even better visual indication of settings from a distance.
The single-impedance output transformer is sometimes a problem. For one thing, you can't as easily disable 4 output tubes and correct some of the output transformer input impedance mismatch by changing the output tap being used. So a 1/3 power switch might not be trivial. But the standard output sounds pretty bad in distortion unless you put in switches for the ultralinear tap and alter the feedback loop. You're probably better to leave it alone or go all the way.
The fact it has a black face makes some people expect it to sound like a blackface, which it doesn't. But cosmetically you have a lot of options. The white trim around the speaker grille was supposed to distinguish it as something special. It looks much better without it. For that matter, it looks good as a blonde with oxblood tolex; you can get away with more cosmetic options because to the black faceplate.