One Fender PRRI is WAY LOUDER than the other

This past summer I purchased a brand new 65 Princeton Reverb Reissue for my son and the tone/volume mix is glorious. I just purchased a used 65 Princeton Reverb Reissue (serial number look up says it's from 9/2008) which has the same speaker and - according to Fender (also via serial number lookup) - the same specs....but it is painfully loud. Like, on 1.5/2 it's louder than the my son's PRRI on 4/5. I can see NOTHING different except one tube being not GT make, but my thinking was that a diff tube brand might - at most - affect tone slightly.

Is it even possible for someone to have done something under the hood to make the amp this loud? The variance between the two amps seems too extreme to be just a nuance of circuitry...but that's why I'm asking the experts here.

It's possible that the taper of one of the amps' volume pots is different from the other one's -- you might check the EIA codes stamped on the housings of both to see if they match. One of them might be a non-standard replacement.

Failing that, you'd need to take a look at the power-supply voltages to see if they match. If they don't then further troubleshooting will be necessary. One possible culprit could be a defective output transformer.

Is it louder with the volume on full or just when it's turned down?