Vertical bi amping Mcintosh MC275 and Rotel RB-1582 MkII

There is a standard, I believe it is 28 dB of gain, which amps often stick to. 

Try it, and see if the balance changes a great deal.

Both @mijostyn and @georgehifi has given good advice. You shouldn’t try bi-amping the two amps on hand without a gain control switch. In your situation where two amps are not identical, an external crossover is a ideal solution for bi-amping to work and sound properly.

Having said that, two things working against you in what you are trying to accomplish,

1. You cannot by-pass internal crossover unless you’re willing to open up the bottom plate and disengage the crossover,
2. B&W loves power, I am afraid the Rotel amp will not be adequate to drive the bass woofers. More than likely, the bass will be flat and dull. 

I’ve been there, done that (used to own 803S, 804D, 800D2). Without tempering the internal crossover, your best bet would be to try the following amp or something similar with your MC275.
https://www.classdaudio.com/sds-440c-class-d-audio-amplifier.html

Good luck!

28 dB of gain on the low frequency or mid-high amp?Is there a simple dial to control gain?

It is done, sound wounderfully wide opened with warmth of tube and tightness of bass.  In this setup doesn't require any external what-so-ever.