There is plenty more practical detail to testing this, but it’s encouraging enough to require a report!
The alternative potentiometer arrived today and I installed it in your DM Buffer, GRJ. To put it decorously, that was a pretty good SWAG!
First, with the gain turned all the way down, a TMCC locomotive still hears its signal and performs normally in limited initial testing, so your hypothesis that signal quality would permit dialing down signal voltage and still give good TMCC function seems correct so far. Haven’t tested that everywhere on layout, but there seems to be room to adjust upward as noted next:
The PS2 steamer is the DCS test subject again. With TMCC gain at minimum and also with it at half-scale (meaning half of knob excursion, not measured resistance) on the new pot, I still get signal 10 and normal function including at a layout spot where there was a “4” and malfunction before with TMCC on the layout. But when the gain on new booster pot is turned all the way up, we’re back to “no signal” or “loco not found”, so the baseline situation hasn’t magically changed.
Since there seems to be at least half of the gain scale available to tune upward for TMCC function if that is weak somewhere on layout, or downward for better DCS function if that is weak in places, there’s room for optimism that this will work out. Lots more to follow in terms of detail of testing both DCS and TMCC/Legacy function in different layout locations, but it’s a great day so far . Thanks for your patience with all the naive questions. Maybe this will help someone else, too. To bring the info down here for someone ready to order a pot, your recommended part is Bourns, mfr part # PDB181-K420K-502B.