Thanks Xavier, that advice helps already, especially about allocating Inst to outputs.
Algo 1, only op1 on all other ops off (I call this an 'init' patch).
Inst 1 is loaded with this 'init' patch as above. Nothing is assigned to Inst 2-6.
Inst 1 has 1 voice, all other Inst set to 0 voices.
Using 1 voice for Inst 1 is louder than using 2 voices.
Inst 1 is assigned to output 1-2.
Switching the unused instruments outputs to other than 1-2 also increases the volume output. With Inst 2 assigned to output 1-2, the output of Inst 1 is quieter (even though Inst 2 has nothing assigned to it).