Does it happen the same way with the boost switch in every position?
You checked that the tubes are good (replace with known good ones)
and that the sockets are ok? We had to re-tension a brand new octal socket on a TAN kit recently, but it was an easy fix and works fine now.
If it only happens with the 8, maybe it is the speaker, if it’s new, you can send it back to Jensen (5 year warranty), they are really good about that.