Just saw this …Passive line attenuator
Probably the least expensive non-DIY way to balance top & bottom centers:
Just put it in line with the center you want to attenuate…. For example if you want less volume from your top center …

Prepro center out —> Y splitting cable
Y out a —> attenuator —> amp —> top center speaker
Y out b —> amp —> bottom center speaker

Simple, no? Happy new year