I'm watching that one myself, for my husband
I LOVE My Bob Marshall. I have a new endurance model and won't ride in anything else! I got on my old Circle Y, which used to be my favorite comfy trail/hacking saddle, and only lasted about 10 minutes, lol. I hopped off and waited for my DH to be done with my BMSS, lol.
You DO need to use a specialized saddle pad with it. Bob Marshall treeless saddles do not have any weight distribution system in them, so you need a good, specialized pad to do that. Skito treeless pads seem to be the favorite. That's what I use and I am a HW rider. I have no problems with sore or fatigued backs, and I am well over 200 lbs
Skito has a full treeless pad
and they have a treeless half pad
. You use the half pad over a thin blanket or other saddle pad. The foam is great at distributing weight and circulating air. You can also easily add shims inside the pad if your horse has any fitting issues (downhill, swayback, prominent spine, etc.).
Here's a picture of my set up shortly after I got it. I switched the stirrups out to padded endurance stirrups (LOVE those on my knees), and I use an over-the-neck aussie/tucker style breast collar for trails. My girth is a 100% mohair roper girth. It keeps the saddle in place well and doesn't gall or make my horse too hot.
I have a ThinLine pad
under my Skito, as I am a heavy weight rider. It helps distribute weight even more and makes things VERY non-slip!