Wow.
As a fail-safe and extra step that emphasizes isolation, (also a mental cue), would be to leave the 1/4" line plugs unplugged from the sockets until transitioning in a deliberate move to mixdown. It's a little more cumbersome, and would be inconvenient for quick switching and "check" mixes.
I'd just say beyond that you could use a matched or suitable recording mixer for the 388, that has a dedicated cue section and tape returns. [But then it's not a 38-388-M1B 3-piece solution].
See,... detail over and over a specific sequence that should be follow, and there's always that quick, inadvertent slip up,... human error. Then,...
shriek/pow!
Of course, I've never blown any equipment with any of the feedback loops I've ever had,... but certainly zapped my ears! The momentary or prolonged feedback can easily blow circuits, too. (Obvious point).
---------
4th choice: use the 388 for inputs/eq/efx and bussing to the 38. Use the M-1B as cue as described, AND THEN MIX off the M-1B directly to stereo LESS THE USE OF THE FULL EQ/AUX/EFF sends you find on the 388.
I'll vouch to a great deal I record with all my tweaks on the input side and going down to "tape", and then mix off a simple mono or 2-buss output less eq and whatever, and I usually get a satisfactory sound.
Thx again.