If the output of the Parasound is louder than the Rouge then you are good. But if the Rouge plays louder, turning the Parasound down won’t work.
I’ve bi-amped with two of the SAME amp, but slightly different minor versions, and felt even that sounded a bit disjointed. So I’d be surprised if a high power SS amp and a high power tube amp meshed well. That said, the Rogue tube amp sounds much more akin to SS amps than you might expect for tubes. And the A21’s gain control is certainly an asset for gain matching the two amps. IF you find the Rogue’s gain to be higher than the A21+ at its max level, you can drop the Rogue down a few dB’s by subbing 12ay7 in the V1 / phase inverter slots in place of the 12ax7’s. It works well; I've tried this. I like the red-label GE 12ay7 from 1962 or 1964, I think (very specific year on the red labels) :)
A 5751 sub its also good for a dB or 2 and sounds nice on its own merits (if you have a good triple mica black plates vintage).