[Robert Gribnau]

I would try to increase B+ to 300 V and keep the cathode resistors at 270 Ohm each.

The maximum anode dissipation of the pentode section is 9 Watt (anode dissipation is the voltage between cathode and anode times the anode current). From the datasheet it follows that with a common cathode resistor of 132 Ohm (which translates to two seperate cathode resistors of 264 Ohm each) and a B+ of 300 V (so the voltage at the centre tap of the output transformer), the tubes operate very close to their maximum anode dissipation (how close depends on the dc resistance of the primary of the output transformer, so on the voltage drop in the primary).