Why High Plate Voltage in 2A3 AMP?

The important spec is the difference betwen the plate and cathode voltage, not from the plate to ground. Many designs use a cathode resistor to bias the tube which raises the voltage at the cathode. If the plate voltage was 2000V and the cathode was 1700V the tube would still only have 300V across it.

I see, thank you, Herman.
So is there any benefit of having high B+, resulting in high plate (to ground) voltage?

Plate voltage isnt' what will kill the tube per se, it's more about dissipation (watts). B+ shouldn't really be a comparison factor if shopping for amps, it's but one part of a large picture. the difference between 320 and 390 is not that large really.

Benefits: sometimes cathodes (filament voltage) are floated higher than typical ground potential, for reasons of noise rejection. This is done with a voltage divider. That might impact where the B+ needs to be in relation, depending on how the power supply is set up. many variables, hard to generalize.

-Ed