If your speakers are fairly efficient and won’t demand much from the amplifier, I’d second the motion that two 20-amp circuits will be sufficient.
However, if your speakers will be demanding a lot from the amps, I’d consider a third circuit, as heavy demands from the amplifier circuits could drag down voltage on the line. Might not be bad to get the other gear off that to another circuit with a more stable feed.
With two circuits on the same phase, I wouldn’t use 240v multi wire, as this wire typically has the two 120v legs sharing a common neutral wire. That’s an efficient arrangement for a 240v line, because the primary legs are out of phase so they alternate use of the common wire.
However, with two circuits on the same phase, both circuits are using the common simultaneously, which means it is double-loaded. Probably not a good thing.
Regards,
Wayne A. Pflughaupt