I have never retubed an amp so just assumed that I should replace all of them. But it makes sense that I could just do the power tubes. It uses EL34s but I am considering the JJ 6CA7s as I have read some good things about them in Marshall amps.
When pricing out all of the tubes for that amp the Tube Store seemed to be more expensive than some of the other vendors (and that was before tax and shipping).