Popping noises

Yes. It could be JJ's. I've had noisy ones. But as often as not with newer tubes it was a seating/contact problem. One of the reasons I suggested that as a probable issue for you was the appearance of the noise during warm up but not continually thereafter. Metal shrinks when cold and expands when hot. When hot the tubes pins would be making better contact.

Gently tap the sides of each tube, in the noisy amp, with the eraser of a pencil. Some tubes have poor internal connections that, "make" after warmup. If you can recreate the noise, or stop it, via the tapping; you've found the culprit.

Thanks again for all the replies. Today, I checked the bias and re-seated all the tubes. The bias was fine and there were no more popping sounds-- the amp sounded absolutely beautiful again..:0)
After a couple of hours listening, I know again why I just LOVE this amp.