The output valves I eventually chose for my redesign of the classic Wireless World Quality Amplifier are the Mazda Pen45. I chose them for the following reasons -
1. 4V heaters in the pre WWII British tradition
3. Reputation as robust and well performing valve
I've not be disappointed with them and have a few spares put aside. There is more information about this valve at the National Valve Museum Pen45 exhibit.
Other users of this valve might be interested to learn that the 6P25 - sometimes considered an equivalent of the Marconi KT61 and Mullard EL33 - was apparently a variant of the Pen45 with the American style base and 6.3V heater. See here for the 1948 6P26 data sheet. Where it states -
Apart from the heater characteristics and basing, the characteristics of the 6.P.25 are identical with the Pen.45.