That sadly has two problems.
The first is that looking at the schematic block diagram for it, the Rumble 40 puts the master volume before the XLR out, not after it. So it will affect the XLR volume too. Assuming this is correct:
The second is less serious but still may be a factor depending on tone preferences; basically Gain is different from Volume. The gain will be before the preamp, and increasing it will allow peaks to overdrive the preamp if loud enough. It can affect tone.