looking at a B+ of just 312 volts, you can try the 7408, 6V6GT, and even the 6EZ5 power tubes,
just remember to re-bias each time you change tubes, 300 to 400mV of bias should do it,
corresponding to 9 or 12 watts of idle tube dissipation....
if you are bold enough, you can go 500mV or even 600mV if the pots will allow you....
for the 12at7, you can try the 12dt8, 12av7, 12ay7, 5935.......
i am modding a pair to run cathode resistor bias so that bias adjustments and red plating will be a thing of the past....
other possible mods is to rewind the opt to run ultra-linear, this way the output stage will be more linear...
still another mod is to regulate the G2 screens to make it stiff, in pentode mode this will make the amp sound better...