B&W Nautilus 804 - bi amping

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

@kraftwerkturbo This is a common path, to use a strong class D amp to power woofers with something lower powered for the highs. The only challenge you will come across is gain matching, as a more powerful amp may also have stronger output gain (not always the case). Generally you can get around this if one of the  amplifiers have a gain control so you can level match. Another option you could pursue is using DSP, like MiniDSP or other solution that can implement room correction for bass issues in addition to level matching. Happy to discuss this more in depth if you need more information!