Never tried the X-Feather but I know Heart of Darkness doesn’t feather on anything I’ve used it on, even newspaper. Bulletproof too.
I’ve done the paper search too and when it changes it’s usually for the worse. Some paper is so smooth, coated, or heat pressed it’s glassy and a FP will just skip. Office store, Target, etc. notebooks are hit or miss for me but I can usually find a pen and ink that works.
Staples will let you buy a single sheet from their copier section so you can try out several brands/types. Sustainable Earth wire-bound notebooks by Staples worked well for me on most pens and inks other than flex nibs. Seems any wet writing flex nib needs FP paper to keep from feathering.
The Hewlett Packard 32lb bright white copier works well for loose leaf paper according to FPN and Goulet.