What contributes most to a change in how an amplifier sounds?

My experience is that the color of the housing is the most influential determinant. A shiney black case produces less harmonic distortion. And I agree that a good on/off switch can make a critical difference. 

In SET amplifier:

1. Output tube.

2. Driver tube.

3. Coupling between stages (transformers and capacitors).

4. Cathodes (cathode or fixed bias, cathode bypass capacitors size and quality, resistors).

5. Signal transformers quality. Output, interstage, input.

6. Power supply. Fist capacitor quality. B+ capacitors quality and size. Voltage stabilization. Power transformers and chokes.

I had a 300b integrated SET amplifier from 2005.

I tweaked a lot of different things:

1. Interstage capacitors.

2. Tubes working points.

3. Cathode and load resistors.

4. Power supply capacitors. Including increased value of capacitors from 100uf up to 3500uf per each tube b+.

5. Cathode capacitors. Including increased value of capacitors from 100uf up to 100000uf.

6. Added interstage transformer for driver and input tube.

7. Added a separate power transformer for drives and input tubes.

8. Build a new 300B SET with external power supply.

9. Changed driving tube from 6f6 to 6v6 and 6L6.

10. Changed output transformers.

I didn’t try to do:

fixed bias, voltage stabilization, SRPP, choke load (LC coupling), direct coupling and many other things.

Hardly any mention of AC cable.  It makes a difference, and it's something users can often change, unlike most of the other components.

@knotscott

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.