Does anybody make...

Your looking for an Active Crossover. What you're trying to do won't work. All the Macs have the outlets they are not separate. If you turn down one pair of outlets you turn them all down. First Watt make two active crossovers, one for OB and a 2 way.  One as a standard 3 way crossover.

STL makes an active tube 2-3 or 4 way crossover. 

In the 70s I ran a pair of C8s (I think) mono tube preamps. 

You can run more than one preamp.. I've ran a pair of C20 one for Bass and one for mids and highs..

Regards

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... 👍