audible pop

[pyonc]

I currently listen to JBL Olympus with McIntosh MC240 as the power and McIntosh C22 as the tube preamp.
Now, all good except for the audible pop through the speakers when I turn off the power amp, with C22 on, first after listening.
The pop is very brief & sharp (NOT from woofers, etc).
I tried different preamps like JBL SG520 and McIntosh C24, but I don't any pop, fortunately.
So, clearly there must be something wrong with C22, though all its functions work alright.
On googling on this, I see something like a DC current leak or bad capacitors, etc of the preamp.
Before I take it to the tech shop, I'd like to ask for your opinion or solution, as usual.
Thanks much for your feedback in advance.

[Kay Pirinha]

Hi,

hard to discern as the MC-240 schematics that is available from elektrotanya.com doesn't show any mains switch at all?!?

Do you have a voltage meter that allows you to measure possible leakage voltages at the C-22's output?

What happens if you turn the MC-240's volume controls fully ccw?

Best regards!