Worse air problem than that. The "baffle" is actually mu-metal plate as
the tip of the input jack is about an inch from the speaker line.
So another air flow mod for this, a full baffle directing air towards
the Zobell, which should also rebalance air inlet to bias towards the
main h/s inlet, a bit like the usual MB problem.
So perhaps internal HF howl-round was the problem. So add capacitance in
the Prea, to downgrade from 20KHz amp, no one has complained before
doing this, a bass guitar amp anyway.
The relay , now desoldered, was seriously buckled and burnt casing so
the contacts were jammed on. As the click/chirp noise had been like that
for a couple of months, 3 times a week use, difficult to believe the
problem was full on ultrasonic instability driven into 10 W of resitors
over the speaker line, all that time.
There is a hole behind the front fascia, for a telltale LED to drive off
the speaker line. What sort of dropper and cap and zener over the LED+ a
rectifier , to come on at elevated amplitude 10KHz and not have the LED
fail at full on ultrasonic "full-rail" oscillation