Popping from tube integrated amp

I've owned an Eastern Electric M520 for about 3 years and have it connected to Omega Hemptones. I changed the EL34 tubes about three months ago from the stock Chinese tubes to NOS Mullards.

This afternoon about a minute after powering up the amp, very loud popping noise came through the speakers - - the popping was constant - about 90 to 100 pops per minute I would guess. The volume was at a (loud) constant level, regardless of where I set the volume control. The popping continued without regard to which input I selected.

After turning off the amp, I reseated the tubes and tried again - - same popping. The tubes all look normal - - no unusual visual cues.

Anyone have any idea what the problem is likely to be? Many thanks for any suggestions.