Listened to both the Gen IIIs, and 4JRs at the same dealership... in differing rooms, with different gear driving each set of speakers.
No 'C' units were ever previewed. As I understand it and think you might have intimated to me the big diff from one another was the networks and/or the components comprising them... higher grade caps, etc.
I'm stunned still seeing the expanse of your own room however. I'd not have felt such results could have been atained. interesting for sure.
I'm done too. Subs are and have been for a while now, the path to improving upon a set system without making replacements en masse.
At near double Tvad's room parameters, I see no reasonable cost effective path for achieving better bass than to proceed with buying a sub (s). naturally, adding another amp of a different flavor/style, likely will help and certainly be a gain of sorts, yet I would be sufficiently fascinated to find out only another amp was enough.
In any case... have fun... and good luck.