Relationship of amplifier sound to transformer quality?

Transformers  are the heart of any amp should be the best.

Jim Stuart of Meridian made a comment years ago when asked about the 'lean' nature of the old Meridian 105 monos. He said what they need is a bigger transformer. The amp and psu are in separate chassis. I just happen to have an extra pair of psu. Could I connect them in parallel to double their rating?

Yes, it's a good idea!

It would be a simple matter of splicing the umbilicals of the psu's and then into the amp as usual.

If you can find the plug and socket, my suggestion is make a "Y" connector so no modification to the original power supplies.

The caps are a pair of 6800uf in each supply so I would up one set to 10,000uf and delete the other?

My suggestion is add a power diode in series to each + and - outputs. Again, no need to modify the original power supplies.

https://www.kepcopower.com/support/diodes.htm

Do you mean at the outputs of the amp/speaker connections? What value would be suitable?

I should point out I'm not interested in redundant operation. I want increased power/current/va rating

Do you mean at the outputs of the amp/speaker connections? What value would be suitable?

No, the diode is in series to each +55V and -55V of the power supplies outputs to Amplifier. The reason of adding those diode in series is to isolate both power supplies and let each power supply only charging its own capacitor.
120V/30A diode is adequate.

I should point out I'm not interested in redundant operation. I want increased power/current/va rating

Since one power supply can provide enough power, parallel two power supply will definitely increase output current capacity and also can serve the function of redundant operation.