JB - thanks again for the thoughts. I'm not an engineer either and it makes me a little queasy thinking about digging into the circuit too much--thus my attempt at a "creative" solution. Would doing a preamp tap disable the normal operation as a combo or can both the line to power amp and onboard 6v6 section be preserved? This dual function is important to me (I don't need an external power amp for practice).
Which brings me to strummin....good point on clarifying my need. I think JonnyBoy got pretty close: "a PRRI that is loud as hell."
But I have a couple of modifications...the basic idea is that I'd like to never have the volume on the head above, say, 3-4--where tone is best for what I'm doing these days. But I'd like to be able to project that tone (or something close to its level of clean) at a high wattage/dB levels, and to be able to do so on the fly, at my discretion, in a live setting. This gets at some of my earlier posts about live sound in general, which I'll continue to improve and experiment with, including just mic'ing the amp and trying to figure out how to make it work in a PA and monitor setup where I don't have control. It's just this kind of volume flexibility + clean preservation I have in mind and which led me down my hypothetical road.
Finally, ideally I would like the solution to not involve diving into the circuit for further mods on the PRRI itself--I'm still a newbie. So, I know it's a lot to ask.
In any event, after further research I ordered the goods to try out my idea this morning (less than $50, so not a big loss if it turns out to sound like a turd). If it's crappy then at least I'm left with the option of doing a preamp tap...or just upgrading the the SMS one of these days. Keep your fingers crossed I don't fry the whole thing!