I've just ordered a Fhoenix Suberpanel dressage saddle and have been using my trainers suberpanel saddle for the last couple of weeks. I love it and more importantly my horse loves it too. What sort of Fhoenix do you have? If it's the prolite version I'm not sure you can use a suberpanel pad underneath... but I'm just learning all this so I stand to be corrected
I have the fhoenix suberpad version and it's good. Suits me and my mare. It looks an feels a bit bulky at first but soon settles down to the horses shape.
If you have a Fhoenix saddle with prolite panels, the suberpad won't be suitable. The Fhoenix suberpanel saddle has the superpad incorporated. I did buy a suberpad recently for a treeless saddle, but didn't like it and sold it one ebay.