Greetings,
If tube rolling between sockets or a different pair of tubes does not solve the problem then It's most likely the stepped attenuator. Yes it's a well known problem from years past. I don't recall the brand but they're pretty expensive. I've not replaced them as of yet for a customer.
Sorry you're dealing with this. Try contacting Audible Illusions and at the lease, do a bit of fact finding.
Regards,
-W