in the JVM1h, I use a 12BH7A in the power slot & it makes the amp more like a 2 watt, w/ more headroom...No master in the clean channel, but volume/gain on the gain channel. So yes, putting a volume control in the loop is an additional master for the gain channel.
If he's wanting to crank the clean channel, yes a volume in the loop would be very beneficial.
12AU7 is lower gain than 12AX7, but AU can push more current. So AU's are less stressed when used as power tubes and can last longer, with about the same wattage output as higher gain AX's.
But preamp tubes aren't really power tubes. A volume control in the loop, attenuator, lower efficiency speaker -- the only way to know which approaches work best for a particular person's ears is to try them.
& in the JVM1, you can NOT swap the ECC82 for an ECC83 in the power slot, as it has a constant cut-out of sound & sounded very bad, like something was broken.