Somewhere I thought I saw something referencing the difference, but do you think I can find it now? If I do find it I’ll post a link. If I recall, the differences are minimal. The Zenith was their original model and then I think the Felix was created to offer a slightly different cut to the panel, or something on that line.
One thing that does show up as different on their website is the Zenith is wool flocked and the Felix is foam. (FYI, my saddle was reflocked shortly before I retired my horse and stopped using the saddle, so nearly new flocking.)
With regard to the D seat, if it helps, I am 5’9" with relatively long legs. My horse did have a rather long back and wide round barrel, an OTTB that was constantly mistaken for a warmblood. Typical high TB withers.
It’s been a long time since I’ve been in touch with them, but I do recall it took a few days before I got responses to my initial emails. I think that was a function of being a small office and, of course time differences being in different parts of the world. Once I did hear from them, they were very helpful; even to the point of calling me on the phone and having an extended conversation to address one of my concerns at the time. Overall I have to say the saddle I have is high quality and has held up very well, as have the pads. I still have the original pads. They are showing some wear but still definatly functional.