The Hypex UCD-400 modules are delivered with a default gain of 26dB, but can be adjusted. The Rogue Medusa adds their own tube input stage, so I don't know if they get increased gain from that, from adjusting the UCD-400 modules or a combination of both. Also, the input sensitivity of the complete amp is 1.0V.
Like you say, with all the variations of sources and speakers ultimately you'll just have to try a passive volume control and see how it works with this particular system, but it sounds like the Medusa is on the right side of the tracks.
Steve