The HT circuit will run quite happily on 70volts to 90volts so x9-9volt PP3 cells will be quite sufficient to power the radio, unless there are issues in the circuit that may need attention causing low volume. If the volume is not up to par then the circuit and valves need to be checked, pay particular attention to any DK96 valves that may be incorporated.
Although general speaking there are very few problems with battery valves, the only fly in the ointment I've experienced, is sometimes the valves go to air (slowly) resulting in slowly decreasing volume. Which leaves you scratching your head wondering why the sound is getting less and less each time you use it, but you know the batteries are OK but it makes you change them anyway
Paul.