Relationship of amplifier sound to transformer quality?

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?

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.

would it be beneficial to increase C to 20,000uf total for each psu, total 40,000uf per pair of psu?

Yes, check dimensions if the capacitors can fit in before ordering. 

One function of the capacitor is to smooth AC ripple voltage on the supply rails, higher capacitance is better, provided its not over-stressing the transformer and rectifier, 10,000uF should be fine but do not go any higher.

Maybe, but I don’t think a soft start on 150VA transformer with 2x10,000uf cap is necessary, IMO.

https://neurochrome.com/pages/the-ultimate-guide-to-soft-start-design

but now they will be doubled up on each channel. Or that will not change inrush?

As I mentioned before, those diode in series is to isolate both power supplies and let each power supply only charging its own capacitor.

They are in parallel which means combined together working simultaneously.

Actually, both PSU will not output exactly same voltages (e.g. one PSU output 55.1V and the other PSU output 54.9V), the one which output higher voltage will do all the work until the current demand from the load is high enough to make the voltage sag to the same voltage as the lower output voltage PSU, then the lower output voltage PSU will start supplying the current to meet the requirements.

Voltage differences is highly expected, that is the reason insert power diode in series to isolate both PSU.
It's perfectly normal one PSU supply current to the load all the time and the other PSU kick-in when higher current is required. That is how this configuration works.