A THD Yellow Jacket is the best option.
read up at THD's website. You can plug these things into any 6L6, EL34, 6V6, 5881 amp and it will convert it into a pure class a machine running off EL84's.
The best thing is to do the mod you simply take your old tubes out, and plug these in. Same to undo it and go back to your origional tubes. No rebiasing or anything.
You can do the mesa SimulClass thing and only replace 2 of the 4 tubes with the YJs or you can go all 4. If you have a 4 tube amp you can pull 2 tubes and have only 2 YJ's for about 15-20 watts.
I love mine.