maybe try running some 6550s in the 5150 first.
conventional wisdom would dictate that running them in an amp designed with 6L6s would be a bad idea (heater current requirements), but there seems to be loads of people doing it - the Eurotubes guy seems to swear by this 'mod'.
you shouldn't need to have a 5150 near max volume either, they are mental-loud amps, must be the way it's EQ'd or the cab.