atmasfere: as a matter of fact, those Sylvania which i have currently installed are extremely microphonic. Are you suggesting it might cause the issue i'm talking about?yes! That very well could be your issue.
Once I had a pair of 7308 tubes that were very microphonic & swapping them with a pair of non-microphonic 6922 made the sonics much better. While they lasted the Amperex 7308 very really superb.