Thanks for putting some thought into this.
First, anyone who lays hands on the THR-1 will discover that not only is it crazy-powerful; but it also had noticeable channel mistracking below ~8:30 to 9:00 on the dial. So IMHO, use of a second volume pot somewhere else in the system is a necessity. If my main system's DAC had a volume pot I'd be doing the happy dance (but it doesn't). So it's down to the PVC or something else like it.
I don't believe the THR-1 poses an overload risk to the PVC, if the PVC is wired downstream of the THR-1. I say that because the PVC, like any volume pot, can "zero out" and, in effect, pass no voltage from the THR-1 to any electrical loads downstream of it.
Of course, if I pinned the THR-1's volume pot to maximum rotation, all I would accomplish is severely curtailing how much of the downstream PVC's volume pot can be safely rotated w/o blowing up downstream loads. I would also almost certainly increase the distortion coming out of the THR-1.
My system DAC has 3.0 volts output, 50% higher than the nominal 2.0 volt output standard for single-ended DACs. That means everything gets even more challenging when I take the PVC + THR-1 out of the burn-in system, where the DAC's output is 1.9 volts. Again, unless I'm maxing either of these devices volume pots, no real overload danger is likely: it's more about choosing which device to stay at steady state vs which one gets to rotate the volume pot.
(see how confusing this gets?)
At the moment I'm testing the THR-1 at fixed volume (~10:30 on dial of its volume pot) w/the PVC's volume pot doing all the rotating. I'm interested to see if the THR-1 will heat up or anything else.
BTW, all this focus on volume pots shouldn't obscure the fact that this amp is a MONSTER--not just in power, but also in sound quality. I can't do for real testing of sonics until I get it in the main system--but so far in the burn-in system w/~150 hrs on it, the THR-1 sounds rather amazing. The bass in particular is nuts...