I haven't. With the MP1 and I spose some extent MP2 the general trend is more gain (e.g MP1 3TM) and high gain tubes. But for bass it's a slightly different story as your primary purpose is not necessarily to drive a tube into clipping/distortion. My take on the MB1 however, is it's designed to keep all that in control so reducing the available gain in the MB1 may not be the best direction to take (no harm in trying though, I'm pretty sure (MJMP, pls correct me if I'm wrong) 12AU7s will swap in electrically without issue). But back to the idea of cranking your power amp and tuning the MB1 down to suit means less gain needed in the MB1 at the same volume.
There are also lower gain variants of some 12AX7 tubes (another way to go....) Also a different tube inV1 and V2 gives more variations....