I wouldn't use a 12at7 unless you really thought you needed to turn down the gain? maybe slot 1 or slot 2.
i don't really know if a 12AT7 would be worth bothering to use. i have had great success with it in my tremoverb for V1 and PI. i have less gain, but its much more musical IMO to a certain degree, and it still has plenty of gain on tap. i stuck one in my MKIV in the PI spot and it helped it clear up a little bit. i will have to give it a shot.
the way i have the rockerverb now, it is just massive for palm muted passages with a single note line (think shadows fall for example).
do you think all JJ's would make it too dark?