My '65 AC 30 sounds better and (louder!) on 115 volts

Did an A-B (with another set of ears present) - amp was definitely louder with same control settings on 115 V vs 120 V. Used a Variac.

Had more "impact" also

Did not expect this.

 

Yes

1965 Vox AC 30 Top Boost. Has a voltage selector and 115 V is one of the choices

 

Mine don't make any sound....

 

Tube amps will start to distort and break up earlier when they are undervolted, or in this case, providing a 1960's voltage. This could give you some of the subjective impression.

We expect more tube B+ rail voltage directly proportional to the wall voltage, and this means more amp output. Without a test tone in and instrumentation to measure the result (vs a guitar, your picking consistency, and ears), it would be hard to make a conclusion in the +/- 2.5% range, especially one that gives results counterindicative to expected operation.