And so this problem continues. After beautiful music for a week, this morning the noise came back. I retightened the socket contacts even more and checked connectivity of every connection going to the tube socket, just in case my playing with the tube in the socket was moving some connection on the socket contacts.
Putting the BeePre back in the system, it continues to be clear that its the position of the 300B in the socket the causes the noise. After playing with the tube sitting in the socket it now is fine again, but I'm pretty sure that at some point the noise will come back. And I borrowed another pair of 300Bs just to make sure it wasn't the tube type. The socket seems to be a very simple set of contacts, but is it possible that the socket itself should be replaced? No obvious cracks or other problems.
