ATTAX Preamp: In Need of Assistance

I just bought a used ATTAX preamp and noticed that once it's warmed up, the volume will randomly start dropping/fading as well as crackling while playing. The volume level will just as randomly come back up to the original volume as well as occasionally higher than original volume. I wondered if the tube was going bad, but I'm having the same issue on the solid state channel as well. I've tried running it out to my power amp and cab as well as direct into my DAW, but it's the same result.

Anyone familiar enough with these to assist?  I'm just trying to figure out if it's an easy fix or if it needs to be returned to the seller as defective.

Thanks.