How about a single FET preamp stage powered by the cathode bias voltage of the output tube? (FET to maintain high input impedance) if you bypass the cathode with a big enough capacitor should be able to get about 8V of pretty clean DC, which would nicely power a single transistor preamp and draw so little current it wouldn't affect the output stage.
Attached image is similar to what I have in mind, not mine, found online, etc, but should be pennies to assemble and do the job nicely without cutting up existing.
Note: may want to put a bit more filtering on that power supply voltage, maybe a C-R-C filter with say 1000uF - 500 ohms - 1000uf just to keep things super quiet