Hi Noel,
I dragged the beast from the heap of "To Do" projects this afternoon.
I must at this point say that this receiver is almost exactly as found.
I had to perform a couple of repairs - as expected the Anode load, and SG feed resistor for the AF driver stage were well out of spec, robbing the anode of nearly all of it's volts.
I also found the AF coupling condenser to be leaking, and whacking around 8v onto the grid of the output valve.
With these three components replaced the audio amplifier seems very lively.
I have tried it briefly in the receiver, and have no problem getting room filling audio on my extension speaker - sadly, it does not have the internal speaker (an all to familiar problem on these sets
)
I will couple the receiver up to some measuring gear tomorrow, and take some true measurements, but I doubt I will find any issues with low output.
Something that might be worth checking on your RX - the AF driver stage has it's cathode grounded via the multiway plugs that feed the IF and RF chassis - I have no idea why Murphy chose to do this, but it might be worth grounding the chassis end of the 220R cathode resistor, and just see if that improves matters.
More to do tomorrow, but I doubt there will be much wrong with your B40.