Tube upgrade for CJ 16LS MKII?

I am blown away by this unit with the stock tubes. Am I missing out on anything? Something out there I should try?
Use the Siemen from or the cheaper ones Mullard at You will be satisfied.
Different tubes do different things, a little more of this, a little less of that. I have always found myself going back to the tubes the units was designed with. Just my thoughts.