Graham -
Since your filament voltage is low, it seems to me that the best way to bring everything up is to change the power transformer primary connections. I'd use blue/yellow instead of blue,and connect brown to black/red, leave white as is. Check things then. If it's too high, connect black and brown back together and tie black/red off. Kicking up the promary should fix your voltage problems. I'd measure the filament and look for 6.3 to 6.5 volts.
... tom