You certainly know your way around the inner workings of the mystery boxes in an audio system and how cool to have you as a neighbor. So much knowledge between your ears.
Seems like the power supply and transformers, no surprise, dominate how an amplifier sounds. I’ve always wondered why amplifiers don’t have a second Power chassis but this clearly would be impractical. Capacitors certainly influence the rebustness of amplifier dynamics and I would guess these wear down overtime.
I, like you, pursued biamping, and I use a MC 901 pair from McIntosh. a more safe approach for me by ensuring added amplifiers Will get along and avoids additional interconnects. I upgraded all the small tubes with NOS telefunken but have continued with the same power tubes I'm not sure how much these will influence sound but quite a price tag to replace them all.
You have a nice size room and a lovely space. My amplifier does quite well in a 20 x 16 room but is deserving of a much larger space.