I'm confused... what's the difference between using a mixer, sending L and R signals to it from the arranger, and then it sending L and R signals to the two L1 Compacts, to simply going out of the L and R outputs of the arranger directly going L to the L L1 Compact and R going to the other?

Gain, maybe? Should be able to match the gain at the L1Compact, shouldn't you?

EQ? Should be able to match the EQ from the T3's Master EQ, shouldn't you?

I still don't get why there should be any need for the mixer... or any difference, for that matter.
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An arranger is just a tool. What matters is what you build with it..!