That sounds exactly like my Ferrograph. The usual cause is the reservoir/filter capacitors (often a dual-type in the same metal can). If they're original they could well be past their sell-by date. As their impedance increases at low frequencies, increasing the current draw by driving the loudspeaker more causes feedback through the DC power supply circuit. Wikipedia describes it as a 'sub-audio oscillator'. In my case, I could see the fluctuations on the HT rail with an oscilloscope.
The problem was solved by replacing the reservoir/filter capacitor with a new one. The ratings of the original are no longer available (8µF/16µF) so I replaced it with a dual 16µF from Cricklewood.
Their selection is here.
So, as Edward Huggins says, check the DC conditions!