New Tubes- Need Burn-In?

You must differentiate between SED's and Svetlana's.

Present day Svetlana's are nothing more than Sovteks in disguise. The really good Svetlana tubes are now called SED and they are made in St Petersburg. The logo on the tube is a 'flying C'. The change over took place several years ago so before buying "Svetlana" tubes investigate.

I use, on different amps, SED KT88's, 6550's, and EL34's and highly recommend them for their tone and ruggedness (I've never had one fail, yet).

I give any power tube 48 hours or so of break in before making a final judgment.

Don't know about a Cayin - probably depends on the sound you are after. You will likely hear a fuller and warmer sound with the KT88's and a more linear sound with the 6550's. I'm running KT88's with EH drivers in one amp and 6550's and EI drivers in another. Both sound great in that combination. So, once you pick your power tube consider how it will sound with the driver tube - makes a considerable difference.

Change the small tubes from the right channel to the left channel and vis a versa. If the popping changes channels you've sourced your problem and potential solution. IME your complaint is typical of small tube deterioration. If that doesn't work change the power tubes in the same manner - if they were matched, biasing for this simple change may not be necessary. If you have to replace one or a pair you will then need to re-bias the new tube(s).