Actually your post wasn't very specific.

Had you posted active with separate amps you would have been told very little issues. A slight volume difference of 3 db. Easily fixed with a knob tweak.

It is impossible to have any kind of EXACT formula, you need to understand every amp is different.

Theoretically, an amp should double from 16 to 8, 3 dB. barely audible actually, to most.

OTOH some amps do NOT double when the impedance is halved, so the difference would be less.
Math is for approximations, reality is for test. And of course the final test/reality is are you happy with it.

i still recommend an 8 ohm, if for no other reason should you want to change to a different model driver, say 288-G, the resale will be crippled by the mismatched pair.