Does anybody make...

I think what you want is a "Audio Distribution Amplifier", which connect among the preamp stereo outputs to the inputs of power amps.

Rolls-DA134-4-Channel-Distribution-Amplifier

What you want is called a trim pot. However I am certain this will be rejected in favor of something inordinately complicated. 

I am not Rube Goldberg.

...time for Audio Heresy <Hour...*L*

One OTW means would be a mix panel feeding 4 amps of varied watts ( the Mac on ’buss’) to your drivers. The 3 (or more?) 2 ch amps could be a distribution amp.

Which in themselves can do remarkable things...or just be amps... ;)

Help I’m a rock’bot.

....since you're just diy'ing for the moment, @sc2 

Good luck, however your approach... 👍

I wonder what a Distribution amplifier sounds like. If you want to go inexpensive use a 2496 Behringer. I personally wouldn’t use it for anything other than bass management. It will however let you work out the kinks in a system a lot easier than anything else I know.. 300.00 or so. Cables and amps are required for every XO point. So Lows Mids and highs. I use a passive on mids and highs.. so it works perfect for me. The subs are servos.

I still used the active XO to worked out the model. It made it super easy to make the passive mids and highs XO.
Mine did incorporate LPads in leu of fixed resistors for DB matching..

I think I understand what you want to do? I think..