[Herald1360]

The barretter will act to a certain extent as a constant current limiter whereas the resistor will allow the set current to follow Ohm's law except for the current taken by any pentodes/tetrodes which are themselves fairly constant current devices.

Whether this accounts for "woolly" I don't know, but it could be interesting to compare HT volts vs heater chain volts for the resistor and barreter at similar ac supply voltages to the set.

Edit: Looked at the circuit, the barretter is only in the heater chain circuit, not the HT feed to get rectifier, so you could be getting usable heater chain current (and lamp limiter glow) at a lower HT voltage than with the resistor which could well have some effect.