Earlier this year I finally fitted a fresnel screen to my main Crown Graphic, this improved screen brightness by 2 stops and makes focusing very much easier, faster etc. (I do NOT recommend the supplier I used !!!). But as you've noticed Loupes & Fresnels don't really go together I find the fresnel means focus snaps in/out easily and accurately.

I use the camera hand-held most of the time (tripods aren't permitted at many places I photograph) and so using a loupe is not an option anyway. I don't have problems with focusing, and there's no problems with lack of sharpness with larger prints 20"x16"+, I use HP5 and normally stop down to at least f16, usually f22 @ 1/100th.

Ian