Regrettably, there are many possible sources. Though the sound is usually indicative of a faulty tube, I've had dirty/poor contacts on tube sockets also produce a similar sound. Rereading your post, did you try swapping the output tubes and then rebiasing? Goodluck.
JADIS DEFY 7 SPIT SPUTTER
I AM EXPERIENCING A SPIT AND SPUTTER SOUND FROM THE LEFT CHANNEL OF MY Defy 7 amp after it warms up for an hour. this is not tube related. When I change left channel signal tubes with the right channel signal tubes, the noise stays in the left channel. All other components in the chain have been checked out. Any advice would be appreciated. thanks