I am also a HW rider and LOVE my Bob Marshall! It’s a treeless saddle, so you have to consider your padding carefully for your horse, but I have had great luck and absolutely love it… I use a Skito pad with a ThinLine pad on top. This works great to keep my horse comfy and my saddle in place!
Now, Bob Marshalls can run long for short backed horses, that’s how I got mine. I ride in a 17" western saddle and got an 18" BMSS (they run 1-2" smaller in “feel”) from a lady who’s horse was too short backed for it (Morab I believe). I got it for $1,000 and it was only ridden in 2 or 3 times (I would have payed $1,400 for all the nifty options she ordered on it). http://www.sportsaddle.com and http://www.sportssaddle.com are two sites to see lots of pictures and all the options. You can demo a saddle too, so you can be sure it’s for you.
Since switching to treeless, my mare’s back rarely gets sore or fatigued, and she has more energy at the end of our rides. I did switch out the stirrups to those padded endurance stirrups. Between those and the simple straps for leathers (no fenders), I have NO more knee pain!
Here’s a picture of my set up shortly after I got it (before switching the stirrups). The ThinLine pad (the black one underneath) had slipped a little when I free lunged her before our ride, the girth wasn’t very tight. It stays put nicely with my rear up in the saddle ;).
http://www.barbarasdomain.com/main/images/photoalbum/12/100_1262.jpg
http://www.barbarasdomain.com/main/images/photoalbum/12/bridg_canter.jpg
Have fun saddle shopping!