Maybe, possibly, possibly not - all depends on how they have implemented the gain structure in the volume circuits or what type of potentiometer they have used. Different amps can have very different volume control profiles, usually they are logarithmic, but some or linear, some have a modified (audio) logarithmic therefore 50% on one amp does not not mean 50% watts, it could be 24%, 50% 75% and anything inbetween. The only constant is that to get 3dB increase in sound you have to double the power and it is generally taken that you need 10dB change to have a perceived doubling in volume. This has no relation to the volume knob setting though.
I wouldn't bother yourself about where the volume knob sits are it is usually has not relation to the watts output unless you know the gain profile and usually can't be compared to other amps. Just turn up the new amp volume knob until you like the sound, simple as that.