If you are feeling spendy, you could go for a larger, higher-voltage plate supply. The max would be 67.5 volts, which would run about 5mA per tube (10mA total) - depending on the filament batter, you can calculate the capacity needed to match lifetimes. In that case I'd drop the plate load from 4K to 3K ohms to maintain the gain. I have a spreadsheet under development for various battery voltages, but it still has some errors in it.
