Musical Fidelity Buffer

It's simply 100% inverting op-amp with tubes that eliminates distortions. It's beneficial for unstable components. Any IC or SS op-amp can do the same job except tubes bring its own pleasant previously mentioned coloration to the system.
As to tech problem ID the channel(make sure by swapping wires) and than replace the tube which don't have to be matched since dual triode being used is already matched to itself. Soldered in-place tube is weird. I used to have X-Can buffer where I could just unscrew chassis and change tubes.

... I also don't understand the benefit of tube being soldered in place. I always thought that tube socket
a)thermally insulating hot tube from the rest of circuit/PCB components; b)provides mechanical vibration isolation. On the long run the solder around tube pins looses flexibility and turns into the cold solder joint. I would definitely place tube socket securely soldered and than place the tube after cleaning tube pins. Maybe it's still good...
You can also verify by retouching the solder joints on each pin.