IMHO, Not really 'better' just a little different. Original review, and some subsequent reviews, describe a less analytical and more euphonic SQ with the '75. I have had lots of time with the '75 (2 pair) and the '74 (6 pair). I slightly, but clearly, preferred the '74 for detail, clarity, and soundstage. However, that is my own preference, no tube acrobatics, just a clean, even handed FR. (Both the '74 and '75, like all the RFL 6N23P, do have an emphasis in the upper bass/lower mids.)
The varied responses to these two tubes has to include the differences in audio chains and individual ear/experience/expectation, but I think it also involves tube variation. My experience with the RFL 6N23Ps is that there is a large percentage of noisy/microphonic tubes, at least 30%, and even very small levels of N/M can alter the SQ performance.
Anyway, in early 2016, I shared a similar curiosity and intent to hear the best available, so I went through a LOT of RFL 6N23Ps to find out for myself just what were the characteristics of this tube. How extensive the effort? Well, at least 200 tubes total from '67 to '74. Single post getters, by my searches, were not available until '74. Did this make a difference in the SQ? No. Same with the silver vs gray shield, no difference in SQ. And of course, if you do a similar search and comparison, YMMV.
Bottomline: IMHO and experience, the earlier year (esp. '70 to '73) RFL 6N23Ps share the same SQ with the SWGP/SS '74 and '75.
So, if you have heard the RFL 6N23P, you already have a sense of the SQ, if not, you can search for a really good pair from the earlier years first, without spending too much.
But I know, you don't really know, until you have had the experience. So you might just have to prove it to yourself. : )
At this point in time, if I do reach for a 6N23P, it will be one of these earlier years. I have, occasionally, gone back for a listen to the '74/'75 SWGP/SS, no change in opinion.
First commented on this here:
https://www.head-fi.org/threads/schiit-lyr-tube-rollers.673709/page-620#post_12569085
You can search my posts for subsequent comments on this topic.
Also, that was the first time I had commented on the RFL 6N3P-E. Actually did not mention it by name, but followed that with this post:
https://www.head-fi.org/threads/schiit-lyr-tube-rollers.673709/page-621#post_12570846
Attention then went to the RFL 6N3P-E. (IMO slightly better top/bottom extension, no upper bass boom/bloom.)
Both excellent tubes, depends on gear and E/E/E.
Edit: I do have a strong opinion and recommendation, and that would be to acquire the best DAC you can before spending any similar amount of money on tubes!