What will raising the Va of atransformers provide?

Larger transformers will have a much larger turn-on current inrush.

Not necessarily bad, but should be considered. Maximum from the wall, even a 20a circuit is what......1800va?

For larger transformer(s) to be effective, this amp will need a dedicated circuit.

Need some more infor. What class is the amp? Listening habbits and speakers will give some idea of actual power needed.

A household circuit is rated for something like 80% of capacity for full time use and only short term @100%.

I don't know rule-of-thumb, but a total of 1000va of transformer will put out a BUNCH of power continuously and still allow dynamic peaks....which are at least 10db and frequently higher than the average level.
Doesn't 10db amount to 10x the power?
Unless you have a HIGHLY reactive speaker, chances are you are playing electrical games with yourself.

Speaker 'efficiency' will be expressed as a percentage. Of power turned into sound, while Sensitivity is the 'db per watt', which also is related to impedance since a watt (voltage measure) is different at different impedances.
Somebody straighten this out if I've made a hash of it.