As far as my knowledge about amps goes, changing the power tubes of an amplifier would require a different biasing circuit(changing out some of the bias resistors) so my hunch is the amp was modded. You should find out if the mod was done by a professional.
I don't know about the Rev+, but I myself have owned a Krankenstein+ head with 6550's and I had managed to modify the bias circuit with some difficulty to have EL34's in it.
EL34's are the more traditional choice for power tubes and have a warmer tone, 6550's have a more aggressive, grittier feel but clean up really well.
Hope this helps!