Thanks to all who responded. The problem is "solved." I tried yet another set of J&J tubes in the pre-amp section and still had hissing and popping. So I decided to take the thing back to the dealer, and replaced all six NOS Telefunkens (two 12AU7 in the pre-amp and four 12AT7 in the amp section) with the original J&J tubes the unit had been delivered with. Hissing and popping went away, and I now have six rather expensive tubes on the shelf.
In truth I noticed a subtle improvement in sound with the original tubes. Not sure what to make of all this but again I'm grateful for the help and suggestions here.
In truth I noticed a subtle improvement in sound with the original tubes. Not sure what to make of all this but again I'm grateful for the help and suggestions here.