Holy crap, what are you using 5 MT-32's for?
Polyphony and compatibility.
The left pair are of the "newtype," 2.07 ROM variety, and are in an overflow-assigned configuration. (That gray unit on top is an FG-10, not an MT-32.)
The right pair are of the "oldtype," 1.07 ROM variety, and are in a split MIDI-channel configuration (via the MTP) due to the buggy overflow mode in the 1.xx units.
A number of games feature custom timbres that utilize certain "bugs" present in the 1.xx ROM units, resulting in incorrect playback with the 2.xx models and variants (CM-32L/64, LAPC-I, etc.). Likewise, some soundtracks/effects will only sound correct when played back on the later units and variants.