[NostalgiaRadioTime]

Thanks, Mike. Oh believe me, I did wiggle the tubes from the top and the bottom with considerable force, enough that it if it was a poor pin connection, it would've made a crackle or changed in sound. I'm confident the 12SQ7 is good. I tried 3 different tubes that are all NOS and tested perfectly, diodes too. No rattles, etc. The vendor I got them from is very reliable and he tests them before selling them (and marks the settings on the box), and I double checked them myself and found them to be as he indicated.

The only reason I thought of needing to change a socket is because these black molded sockets cannot be serviced. if the tube is too loose or not making a good connection around the pins, there's no way to get inside to crimp the hole tighter.

But I will try your suggestion with the 1nF cap tomorrow. Thanks!