What will raising the Va of atransformers provide?

If your amp runs in pure class A, there is no benefit in upgrading the transformers besides slightly less internal heating of the transformers.
If your amps are the more popular class AB or AB1, depending on how good was the design to begin with, you may hear more dynamics (increased headroom) with larger transformers.
You will certainly need to upgrade the rectifiers, because a higher wattage transformer usually has less series impedance, therefore it will deliver more instantaneous current to the rectifiers during the power up cycle.
Also, you may need to reduce output stage bias, because an over-specified transformer may output up to 10% over-voltage. In summary, this is not as simple as it seems.
Here is a good read about choosing a transformer supplier;
http://www.powertransformer.us/choosingcustomtransformermanufacturer.htm

Thanks, Stanwal, excellent explanation.
Class A amps are fully on (conducting 100% of the rated power) most of the time, therefore the power transformer, rectifiers and power supply capacitors must be over-specified, in order to supply the high current demand, stay relatively cool, not buzz or fail.

On the contrary, Class AB amps pass a fraction of the maximum power most of the time, while playing music. On musical peaks, a larger transformer and stiffer power supply MAY help output more peak power.

Some amplifiers are current-limited by the choice of output devices or the size of the heatsinks. In that case a larger transformer would be of little help.
I suggest that the OP contact the amp's designer for guidance.