The easiest up grade is put in premium EL-34 's. I have found the best results with Groove tube GT-E34 LS tubes.
I have owned most of the mods, but probably the best and most consistent(IMHO) was the Audio Research, as described in Audio Amatuer and I know there are one or two companies out there that sell the kits or do the mod, may even be Welbourne Labs. I also liked Van Alstine's, but it was one of the older units and I am sure has changed. Would also maybe perhaps ever so slightly suggest buying one of the mods that are out there used, cause, stock Dyna's are just getting scarcer all the time, and I am almost to the point that I hate seeing them modded. I have seen used AR Mod ST70's go for 5 to 6 hundred, and I don't think you could get a stock 70 turned into one for that.