The "super twin" and sister "studio bass" combos are not popular for guitar, perhaps the most despised of non-reverb fender "twin" amps, with ultralinear silverface sound in a blackface amp disappointing those who expect it to sound like it looks. The later "super twin w/ reverb" is not any more popular for having springs. No love from guitarists looking for some dirt, even the pull-distortion feature sucks. Huge trannys make it top-heavy, huge speaker magnets make it an inconvenient combo. Usually seen with broken knobs from falling over.
BUT if you put the chassis into a head cabinet, it's a GREAT bass head, or good for pedal steel. The ultra-linear operation doesn't make for good distortion. But as a head it's also a wonderful extension for any other amp. For me, it's a mini-svt head and the big trannys make good bass. As a head it's still huge but portable. And 180 watts drives inefficient sealed cabinets well if you like that sound. It will easily push an SVT bottom, or 4 to 12 12's, or 4 15's. As a combo with 2 12's it's just plain crazy. The Motor City Madman used 6 super twins onstage, but even he had enough sense to plug an additional bottom into each of them.
It's common to unplug or switch off the cathode connection to a pair of tubes in a parallel push-pull amp, making a simple push-pull. But you usually minimize the impedance mismatch at the input to the output transformer by altering the speaker impedance load or switching the output impedance selector to use other output taps. The super twin doesn't have selectable impedance taps because it was meant to be a combo. Cutting power in half doesn't alter the volume much, it just brings on the dirt a bit earlier. You have to cut power a lot more to have an effective volume to reduce volume or get preamp dirt. And you really should only turn off one pair of outputs and with 6 output tubes that 1/3 reduction in power doesn't translate into much noticeable reduction of volume in decibels. You'd have to also add some power soak resistance and lower your B+. To cut the perceived volume in half you should cut power to about 1/10th what you have now.
Now on my Peavey Classic 50, turning off half the EL84's makes a pleasant change, bringing on the el84 output dirt as opposed to the 12AX7 preamp dirt. That's significantly different from the super twin.
Does yours hum a lot? If not, just use the master volume. If you need to to reduce hum & hiss from the power amp section, try a power soak load.
Good luck, I'm just starting to play with mine. This is one of the few times I don't feel bad at all about changing an old Fender. IMHO it just makes a lot better head than combo, much more versatile and movable. And downright pretty IMHO, compared to the rather ugly black combo. I'll post pics when I get a chance.
Perhaps if you could add high-voltage regulators and some impedance network at the input to the output transformer and switch in some output load you could turn off 2 pairs of output tubes and make the change noticeable. But then you'd want to reduce the negative feedback and add a switch to turn off the ultralinear mode. At that point it might be more practical to get a second smaller specialty amp. There's some very nice kits out there.