Can you unwind a few turns of the filament winding from the
transformer? If not, then perhaps you can add a few turns and connect
in series with the current winding? Depending on how you connect the
two windings you will either add or subtract the voltages.

Another possibility would be to connect two or four diodes in an
anti-parallel connection in series with the filament circuit, that
would subtract one or two forward drops, but would perhaps introduce
some unwanted harmonics into the circuit.

Thomas