[Arran]

You have to remember that with a three way portable the filaments are operating off of the B+ voltage coming off the cathode of the rectifier tube, with a wire wound dropping resistor inbetween. The rectifier tube is usually something like a 117Z6 or 117Z3 because the filament voltage of those tubes has a working voltage of 117 vac. For starters the voltage drop from the line across the tube is anywhere between 10 and 20 volts, depending on current load, then you have the filter network of capacitors and resistors, then the dropping resistor which drops the voltage down to 7.5 volts for 4 tubes, or 9 volts for five tubes.
An extra 6 volts at the line may translate into .5 volts on the filament string. To be sure I would try measuring the voltage across the filament string, if it's a little too high then add an extra dropping resistor calculated according to Ohm's law, or if you don't mind some "sand" in your set add a silicon diode in series with the tube filament string which will knock about .7 volts off.
Regards
Arran