Hello Bernie,
On the Paramours, there should be a 249K resistor from grid to ground. If you use a passive volume control with very short cables to the amp, the highest value I could see being sensible would be a 50K control.
If you remove the 249K resistors and plug in your volume pot, you'll now have a situation where if the RCA cable slips out of the amp, the grid of the 12AT7 is now floating, and you risk tube damage. I wouldn't do this, and having the 249K resistor in there isn't going to load your passive control unreasonably.
There's also the grid stopper, which is a little 220 Ohm resistor between the RCA jack center pin and the grid of the 12AT7, and that also has to stay.
The Stereomour has the passive volume/balance controls acting as a grid leak resistor, but they will stay in the circuit regardless of whether anything is plugged in.
-PB