Normally one would work from the circuit diagram, and check all the posted voltages with a high impedance voltmeter. However, you could try swapping tubes out, if you have spares. What tends to happen in these old radios is that the capacitors dry out, start to short and sometimes burn out or damage resisitors.
If you are asking if the impedance is the problem I would think not. I don't know that particular model but 8 ohms was uaually pretty common.
