I used JJs (pre and power) in the 400+ I used to own. They sounded great, much clearer than the stock Mesa tubes that were in it and they were far cheaper.
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Originally Posted by knigel
Running through a field in your underwear on an episode of COPS barely qualifies you as "being on TV."
It's a fixed-bias head: you can swap the pre-amp tubes without much issue, but the power tubes will be a different story. You should take it to a tech and have him add bias controls. Then you can do whatever you want.
I did the JJ 83 (I think?) pre-amp tube mod (featured on this forum) when I had it and it was a wonderful idea. More gain control for sure.
Tubes for Mesa 400+ 
