Solved thank you!
Find our what tree is in the Stubben and get it in a dressage saddle?
Does the dressage saddle have a point billet? If no, I would start by talking to the fitter about whether that might help, and then trying different girth options. I have a pony who is prone to saddles sliding forward even when they fit - the offset TSF-style girth didn’t help him, but a crescent shaped girth did. On a different saddle, the crescent didn’t work but one of those ‘humane’ equalizer buckle type girths did the trick. And a point billet is basically a must for him, unless I want to ride his neck 
