Hello:
The Dr. knows best…
IMO, Probably not the bias pot.
The bias supply may be tapped out due to a panel issue.
If it works and sounds good, turn up the bias control and see if these is some discharge over the next several days, or with a change in higher humidity/ weather low pressure. This may show you are close to a reasonable bias is level. Then enjoy.
Or,
A speaker turnaround return will take time and based upon repair and inevitable updates will run you some funds. you should then consider updates all things considered.