I've run mine every which way. Makes no difference. Subs are mono so either channel same result. Input impedance typically 20k to 50k ohms or more, so no load on the amp and again which channel either one or both doesn't matter. So get some wire, hook em up, don't overthink it.
Subwoofers: speaker level vs line level
Although my sub accepts both line level and speaker level input, I have only ever used preamp's unbalanced line level out to sub as all speaker terminals are occupied with bi-wiring the main towers via banana connectors. I know that REL, maybe others, recommend speaker level connection, which I wouldn't mind trying (not that I'm displeased with present setup). Would there be some way of doing this other than bridging the speaker inputs thereby freeing up terminals to run subs? Or, if continuing to bi-wire, switching to spade connectors for towers while using bananas to sub? (Amp does offer loop out, but that is line level). Assuming these approaches would work, would there be enough benefit to justify the hassle?
Because Output transformer in the tube amp is the limiting factor of low frequency extension if connect subwoofer via speaker level connection.