IDK, but you could use a passive splitter to turn mono into stereo and add gain at the TR-06 input or use a small powered mixer like one of the Moukeys that can do mono to stereo and gain. See thread below for detailed discussion. TL;DR: cheap & good. You probably don’t need the big one for your application.
Also worth considering the RD-6 if you just want the drums & basic sequencer for cheap. The TR-06 should have a much better sequencer and probably more drum editing options. (Also consider the 6s, which is all of the TR series in one box)
The 8s and I assume 6s manual includes a detailed signal flow schematic, too bad the Boutiques are so light on documentation.