Does anyone have a schematic?
This doesn't sound right to me.
The amp can take either balanced or unbalanced inputs.
And it will drive either balanced or unbalanced outputs.
I'm guessing the OPA2604 is an input stage Op Amp configured to accept either balanced or single ended inputs which converts the signal to single ended which then drives the 4 OPA604 Op Amps which are for the 4 output amps:
Left Hot (or non-inverting)
Left Cold (or inverting)
Call it an educated guess.
Single ended output is probably just the Left "Hot" and Right "Hot" amplifiers.