Smallbox combo Volume issues

Just received my Smallbox combo from Sweetwater and love it. Volume starting cutting out and sounding thin. Could this be a preamp tube and if it is which ones are recommended. Is there a Diagram on how the tubes are set in the small box combo. Thanks for your help.

I run TAD el34 B in the power section, Ruby preamp tubes, and Tungsol in V1. I think they ship with a JJ in V1. I know it did when i had it. I think the Tung sol is a huge improvement in v1. They may not ship with a tung sol because of reliability issues. I may be wrong.

Thank for your help