So far I have swapped out the two rectifiers for 5AR4s (JJ Electronics), replaced the 1.5H choke by a 3H Hammond one, added two 10R/5W and one 50R/5W resistors into the power supply plus a diode behind one of the 5AR4. So far so good, all's working as it should, getting around 400V on the plates of the 300Bs, and waiting for Skunkie's recommendations on the output transformers. My only worry right now is the 50R/5W resistor as it runs pretty hot at around 150 degree Celsius (8.4V / 0.17A / 1.4W). I'm building a Variac + a dim bulb power supply as my mains runs at about 245V (instead of 230V) and that may not be that great for the amp a there doesn't seem to be any voltage stabilization in the circuitry (as far as my layman's eye can see).