A fitter can explain this better probably, but my lay person understanding for my horse is that he needs a MW tree, and then the dropped/K panels to provide the correct contact with his back. They larger panels will also help distribute the weight more evenly on the portion of the back which is intended for weight, instead of being concentrated close to the spine with standard, narrower panels.
I’d definitely go Passier for a big shoulder/high wither, especially if the topline needs a narrower fit. The shorter points of a Passier tree give soooo much freedom to the shoulders.
My cob has a 42 year old Passier for dressage, set to Passier’s Wide (I forget the specific number) and it fits him as well as, if not better than, his XXXXW Santa Cruz jump saddle. (Also gives a good example of the extreme variation in tree sizing between brands).
If my precious Passier ever meets an untimely demise, I would not hesitate at all to get another. This thing is old and what many call ugly (flat, brown, short biillets, no rolls/blocks, worn suede knee patches on flaps) but it fits me and my gelding better than anything. I fully intend to take it to my grave, haha!
Edit button isn’t working.
My friend has an OTTB who is going okay in a mid-90s Stubben. Nice high/cutback pommel. However there isn’t the same freedom in his shoulders in it as there was when she was borrowing my Passier. Better than the Santa Cruz or Collegiate she tried though, at least.
Short tree points
I had a similar warmblood with a mountain of a wither and big shoulders (he was 17.2) and he went really well in a Sommer Diplomat. Sommer’s come with trees with short points for shoulder clearance and are adjustable trees (by a saddle fitter) and so a similar saddle may be your best bet. Schleese’s also have similar features and often cutback pommels for the wither.
Good timing! I have a young TB with a similar build and have been thinking of picking up a Thorowgood or Kent & Masters. Those of you who are familiar with these saddles, which I gather are the same apart from synthetic vs leather, how much deeper is the rear gusset in the high-wither model versus the regular? My TB has a decent wither and some moderate hollows so needs panel support there, but as his back line rises gradually toward his croup too much paneling there can result in a tipped-forward seat. I suppose if they are fully flocked my fitter should be able to sculpt that a bit and take it down a bit if needed.
Also, don’t wish to hijack your thread OP, but have one more question about these saddles that I haven’t been able to answer after trolling COTH. I like the look of the external block available on the Kent & Masters S series, but I have a long femur so blocks are rarely in the right spot, especially because I have a hip problem that prevents me from riding with an extremely long stirrup. In my ideal world I would get a custom or semi-custom that has blocks on a forward flap. I would love to hear from anyone with a long thigh that has ridden these saddles and can comment on their experience. The movable block probably makes more sense as it is more versatile and could be swapped out for something different entirely, but I have an irrational hatred of velcro despite its practicalities. 
I seem to recall seeing a COTH post from someone around 6-feet tall that rides the K&M saddle with the external block, but of course cannot find it now that I’m hunting for it…
[QUOTE=visorvet;8986434]
Good timing! I have a young TB with a similar build and have been thinking of picking up a Thorowgood or Kent & Masters. Those of you who are familiar with these saddles, which I gather are the same apart from synthetic vs leather, how much deeper is the rear gusset in the high-wither model versus the regular? My TB has a decent wither and some moderate hollows so needs panel support there, but as his back line rises gradually toward his croup too much paneling there can result in a tipped-forward seat. I suppose if they are fully flocked my fitter should be able to sculpt that a bit and take it down a bit if needed.
Also, don’t wish to hijack your thread OP, but have one more question about these saddles that I haven’t been able to answer after trolling COTH. I like the look of the external block available on the Kent & Masters S series, but I have a long femur so blocks are rarely in the right spot, especially because I have a hip problem that prevents me from riding with an extremely long stirrup. In my ideal world I would get a custom or semi-custom that has blocks on a forward flap. I would love to hear from anyone with a long thigh that has ridden these saddles and can comment on their experience. The movable block probably makes more sense as it is more versatile and could be swapped out for something different entirely, but I have an irrational hatred of velcro despite its practicalities. I seem to recall seeing a COTH post from someone around 6-feet tall that rides the K&M saddle with the external block, but of course cannot find it now that I’m hunting for it…[/QUOTE]
I have a Thorowgood T6 standard model that I like but it has a very shallow rear gusset. Unfortunately it sits too low behind on every horse I put it on so I have to use shims. I don’t think Thorowgood/K & M have very deep rear gussets in general
I use the Custom Saddlery laguna in a medium wide for my wide shouldered with withers OTTB mare.
I second the Fairfax recommendation someone else made. Has front gussets and was recommended to me by a saddle fitter after I sent him the specs for my horse’s withers and back, which are exactly as you describe yours.
Saddle-fitter called the back “angular.”
When you ask for size of tree, do you mean width? Fairfax is one of the ones that uses the Simatree system, so width is adjustable. I have a collection of bars now, for use on different-width horses.
The length of the tree (my saddle) is 22," for a 17.5" seat.
Length of tree can be made shorter for that size of seat if you want to pay extra for custom-made.
Since I can’t seem to “Edit,” let me add that if you did get a custom-made shorter tree, the padding would not, IMO, be in the right place to absorb shock correctly.
(I don’t think Fairfax will do this kind of customizing, but Black Country would).
FWIW, I have a horse built like this and the Hastilow I have, which is built on the same tree as the Thorowgood/Kent and Masters/Fairfax does not fit him particularly well in the shoulders even though it has the front gussets. I think the tree points are too straight.
Now that scruffy mentioned it – I did have some problems around this too. It requires using the wider bar and then some shims at various places under the saddle.
Not sure any saddle is conformed with a double-curved tree. This one of Simatree’s is already called an R shape. (Sometimes ‘K’, sometimes another letter I forget).
