Is the very high gain phono circuit fed into the line stage then out? Many older tubes can have inherently higher noise levels. I guess it is possible that the old tubes aren't really quiet and when amplifying each other (as I have supposed is possible) the noise rises to audible levels. Just a guess.
Tube Interaction in Counterpoint 3.1
I have a heavily messed-with Counterpoint 3.1 pre that I've repaired and mostly put back to original spec. It uses a pair of 6DJ8s for the line preamp and one 6DJ8 as the phono pre. I've been doing some tube rolling and discovered that when I put good Amperex tubes in the line level sockets AND the same in the phono socket, the phono tube crackles and hisses badly. When I replace the line tubes with new Russian tubes the old Amperex in the phono spot is fine! I have 6 or 7 good European Amperex tubes and they all act the same way if they're in all three sockets. They work fine in the two line level sockets no matter what's in the phono socket, but I have to use new tubes for line if I want to use an old tube for phono. What the hell! I've never seen anything like this. Any ideas?