Thsalmon don't buy anything recommended here unless you hear it on your SY table. Cart/arm/matching is very hard to predict. Add to this that certain products become fashionable - more because they are different than a genuine step forward. Plus, not everyone here pays attention to and values the same characteristics.
FWIW I heard GT Audio's Schroeder Ref/Allaerts recently and found it strident and fatiguing. But then I'm happy to trust my own ears and I don't believe that there's rarely a correlation between component price and listening pleasure.
I think the Schroeder looks beautiful, however I've never been fully convinced that it handles amplified music as well as acoustic genres.
The S7 tonearm is a unipivot so I would suggest you start with your Decca mounted on that. Your second arm therefore should be gimballed and the obvious contender is the Triplanar VII for ultimate adjustability.
My other shortlist nominations, based on engineering quality/design integrity are the Breur Type 8 Dynavector DV507 MkII and Ikeda IT-345. I don't like the SMEs too much - I think they overdo bass when mounted non-SME tables, the wiring isn't the finest and I have doubts about cast arm tubes - I think rolled tubes sound better. YMMV.
I have had a DV507 Mk II in my system over the last month and it's a beautiful piece to use - it doesn't get in the way of the music, although it doesn't do 3D imaging (but my room is acoustically treated to minimise HF reflections anyway). It doesn't have the air of a Schroeder but I could live with its compromises more readily.
Also try the KJP in the S7 arm too and if it's synergistic I would suggest you leave it there and add a Hadcock arm for your Decca. Not the sexiest or most expensive component but guaranteed to sing with a Decca fitted.
You have an excellent system and the credentials to be able to have your choice of tonearm on home trial, so again, don't buy what you haven't heard.
