Another thing to do before you spend any money on tubes is check if the humming is coming from one of the transformers. This is a very common problem with transformers; when they built the transformer, they dip it in varnish to "glue" tha laminations together so that they don't hum when powered up. Unscrew the rear panel and stick your ear right next to the transformers; if you can localize the humming at a transformer, you can either have it replaced, or, if you're adventurous and inderstand that this amplifier has a power supply with storage caps that can kill you if you touch them accidentally, you might try re-varnishing the laminations yourself.
And hey - IMHO, anyone that's owned a twin and has played for 20 years could probably GIVE lessons... you can't hide a lack of talent or technique with a clean amp like a twin.