I am guessing the PSU you found is a switcher. If it was me, I'd not encase this inside the same enclosure as the amplifier itself. It's asking for troubles.
As for powering the amp from a lower voltage... Why would you even wanna do that? Tubes benefit from more voltage, not less.
It'll work, but you'd have to recalculate all the resistor values. I currently have my output stage working at 25 V just fine. Though the tubes are powered from a 138V rail.