Audio Research CL-30 Bias Drift

A client brought a CL-30 in for a re-tube because the amp blew a fuse when a tube shorted. While doing my usual voltage checks without tubes installed, I noticed that each half of the amplifier has a tube socket that has a tremendous bias drift that slowly drops from -50 volts to -19 volts. The other two sockets remain at around -49.
Not having studied the amplifier schematic, does this sound like a common problem associated with these amps? This would certainly explain why it cooked a 6550. Does anyone have any suggestions in sourcing the problem? I build and repair vacuum tube guitar amps, so this sort of hybrid circuitry isn't what I see very often.
Thank you in advance.
Billy