Have you got a
‘scope yet Steve ? If
you have then use it
to check for any HF
oscillation on the
speaker output and/or
output tube plates as
the pot is turned up
and you hear the
thump. (You will
need a high voltage
probe to look at the
plates.) If you don’t
have a ‘scope
available connect a
DC voltmeter to
monitor the B+
voltage and if you
can see a sudden sag
on the B+ when you
turn the pot past the
thump point then you
probably have an
oscillation. 

Dave