Hi Reeps. My preference for current production EL34's is the Shuguang EL34B STR. tubesandmore has them sporting the Ruby label.
As far as the 6GH8A to 7199 socket adapter I haven't used one yet but my reading indicates that it works fine. I'll be needing a couple eventually for one of my Ampegs too.
btw, that RCA GZ34 could be a relabeled Mullard and if it's working you can keep using it. Same with the preamp tubes although it's fun to try different ones and wise to have backups. I do like to start fresh with power tubes unless they are some prized vintage type (that would most likely end up in one of my guitar amps anyway)