Choose the amp that lets you. I have an Amp/One A that has level controls on the front. I know that Crown does, and some Rotel amps do as well.

Get an SPL meter and hook up the on board center channel and take a measurement. Then unhook it and hook up the external amp center. Adjust the amp level so the the SPL measurement is the same as what you got for the onboard. Bob's your uncle.

Anthem: AVM60, Fosi DAC-Q5
Axiom: ADA1500, LFR1100 Actiive, QS8, EP500, M3, M3comp, M5