I've got a 'Magnum' neck and use it now & again on my 66RUL with no pitch problems or any other problems whatsoever. To me, it's brighter, more focused than the 'Super VI' (and the previous poster said just the opposite, one player's 'spread' is another player's 'focused'... as I always say) and certainly has more volume, no change in response from the horn or using my air in any different way.
This may be of interest to those that have had problems with G3 (first altissimo G) as I did: That G was very difficult to play at first, I have never had any problem with any horn, neck, even mouthpiece playing the high stuff, but I found that high G to be quite wild on my P. Mauriat (with the standard issue Super VI neck) at first. I had the guys at the local shop have a look & listen, they put a different shaped pip in the neck and shut down the front F adjustment quite a bit, which made that G speak with ease. I've never had a problem since and advised a former student of mine (whom I was pleased to help become a P. Mauriat endorser as well) who had one of their 'Influence' tenors, he was having the same difficulty, took my advice with the front F adjustment (but didn't change the pip in the end) and 'all is well' with his G3 as well.
I'm also using a different fingering now and again, the B key + side Bb, as opposed to the usual front F + Bb side key. This also speaks much better (for me), has a slightly different 'texture' with the different venting and the pitch is fine (for me).
I've really enjoyed the Magnum neck, I use it when I need a bit more 'jump' and brightness from the horn, but I prefer the Super VI neck overall.
thanks, enjoy your horns! DC