Schipo's correct.
Swapping the interconnects at the linestage will tell you if the problem is with the preamp or in front of it. If you swap the ICs and the problem still occurs in the same channels, then it may be a tube (or tubes).
Swapping the interconnects at the linestage will tell you if the problem is with the preamp or in front of it. If you swap the ICs and the problem still occurs in the same channels, then it may be a tube (or tubes).

