I am another fan of the Passier GG for my Tb. She has shark fin withers though so she may have a little different back than the horse that you describe. A saddle fitter that used to be in our area said that he could adjust the Passier tree one-size down or up if needed. So I bought a medium tree on e-bay, and he narrowed it for my horse. It still fits her great even though she is now 18 and not ridden nearly as hard as when she was competing.
I bought a Kieffer Wein Pro on ebay for my previous OTTB and it fit him like a dream. Current OTTB is a bit more WB in build and has wider shoulders, so the Kieffer did not fit him at ALL. He's in a Kentaur, which I love, and which has an adjustable (by a saddle fitter) tree. Kentaurs are also reportedly a fairly versatile saddle that fit a lot of types. They retail new for approx $2500 tho.
Really wanted a Niederssuss, which also fit him well, but was concerned about the non-adjustability of the tree since he is only 4 and changing all the time.
Have heard Albion and Prestige saddles also tend to fit TBs well.
Per our saddle fitter, Passier's also usually do although he whipped the one he tried on my guy off like it was on fire..lol. He's just not quite your typical TB build.
"Show me the back of a thoroughbred horse, and I will show you my wings."
I've bought and consigned several saddles with UsedSaddles.com after getting hit with unreturnable saddles and buyers backing out on eBay. I do like their generous trial period and the fact that they'll take a saddle on trade-in.
They seem to have several of the saddles mentioned here in stock.
My County Competitor worked well on almost every horse I rode...from the slab sided TB to the more round warmblood.
I love my County Competitor. It has medium knee rolls and a seat that isn't too deep but I wouldn't say flat. I am also 5'9'' and I am all legs (about a 35'' inseam), and it is very comfortable for me. I have a medium tree and it fits my OTTB mare great as well as pretty much anything I put it on, though I never really ride something super wide. But it has been on warmbloods and thoroughbreds and seems to sit well on everyone.
Have a 5 yr old OTTB with larger than average withers (so he needs clearance but still settles in a M or M/W tree) although not wide. I have a County Perfection. I tried 8 or so dressage saddles and this was the only one that actualy fit him really well so I am eating ramaen noodles for the rest of the year but our rides are nice
I've also had good luck with the Passier saddles on TBs. Both jumping and dressage. My mare currently goes in an older Passier dressage saddle I got used, since I also don't like a lot of stuff and prefer a shallower seat. She also went well in the Kieffer Niedersuss (sp?), so I'd definitely give the Kieffers a look. I liked that one, but couldn't beat the price I got on the Passier.
IIRC, when I was shopping for my guy, same built, the Passier did fit him well, but it did not fit me that well. I HATED sitting in it and I'm really not that picky about saddles, but I don't want to be miserable, LOL.
Look into the Walsall saddles (Black Country, Frank Baines, Kent & Masters). I bought a K&M (I own one BC, but that's for my curvy horse, wanted the Baines, but too pricey) for the OTTB and the tree fits him like it was built for him. I did get a bad dye lot on the flap leather but the company is great to work with and are taking care of it. That lot has been resolved and others I know who have them have had no issues. Oh and I LOVE riding in it!