I have a near new Mesa 400+. I have searched and read up on the pros and cons of the various tubes. The only alternatives available to me here in New Zealand are JJs, and the dealer is able to match them to Mesa specs. I know the JJs would be an improvement but my question is - would you wait for the Mesa's to wear out or replace them now? I would replace all the tubes, pre and power amp. Someone who has done this might be able to tell me how significant the change would be and how the sound will change. I run it through one or two SWR Goliath 3s and play mainly blues using a 69 Fender Jazz or 68 Telecaster bass.