Joe:
I've owned the Premier 12s in both power configs, as well as the 8's in both. I preferred the triode versions IF the speakers were relatively efficient. I actually compared the 12 and the 12xs side by side for about 2 weeks and came to the conclusion that the xs version had more air and seemed slightly more natural in timbres than the 6550 version. I ultimately kept the 12xs and sold the other pair. My experience with the 8's has been that the bass control with the 8As was noticeably more authoritative than the 8xs.The xs versions of both amps have a sweeter midrange, but are slightly rounded at the extremes. Again, I used the 8xs more than the 8As.
One other consideration, the EL34s are cheaper to replace and produce less heat than the 6550s. Really, you can't go wrong with either version.
Jeff
I've owned the Premier 12s in both power configs, as well as the 8's in both. I preferred the triode versions IF the speakers were relatively efficient. I actually compared the 12 and the 12xs side by side for about 2 weeks and came to the conclusion that the xs version had more air and seemed slightly more natural in timbres than the 6550 version. I ultimately kept the 12xs and sold the other pair. My experience with the 8's has been that the bass control with the 8As was noticeably more authoritative than the 8xs.The xs versions of both amps have a sweeter midrange, but are slightly rounded at the extremes. Again, I used the 8xs more than the 8As.
One other consideration, the EL34s are cheaper to replace and produce less heat than the 6550s. Really, you can't go wrong with either version.
Jeff