Glad to here that this is not a crazy idea of mine. I will first try to drive the B&W 604 with my 4 channels of my Onkyo TX-RZ810 (output test showed around 180w/8ohm/260W/4ohm, 2 channels driven) for simplicity reason. If the low impediance is causing problems, I may use the Onkyo initially for mid/high, and a class D for the bass.
The problem with gain matching of the seperate amp is caused by a different 'gain rate' of the second amp, i.e. I can use the volume control to 'set' for perfect balance, but chaning the main level may result in too much or too little gain for the external amp (since the gain curves of both amps may differ). Is that a correct understanding? The external amp then (if gain curves differ significantly) would need an adjustable gain (not to be mistaken by the 'volume' knob)?
For discussion purpose, does any of the 2 class D brutes shown in the photo (Behringer EP2500 an dQSC CX702) have such 'gain adjustment'?
https://drive.google.com/file/d/1GcLUHNY5U9AAImAvQYa4RHmLBioMabfk/view?usp=share_link