Popping sound

Before switching tubes, try switching the speaker wires.

After that you can switch the interconnects from pre to amp

Good point about speaker wirr interference .   I had a ethernet cable running alongside a speaker wire and it caused a weird intermittent crackle noise.  

Although the sound probably originates with a tube in your preamp, often there is nothing wrong with the tube, just the contacts with the pins and socket, and the problem can go away by just wiggling the tube or taking it out and reinserting it. Reversing the tubes can also solve or identify the problem (if it's a bad tube).  

Ask Chat GPT. No, really. They have an audiophile network that has been incredibly helpful in trouble shooting problems like this. I was shocked how good the response was.

Thanks for all the responses.