For $500, you can do way better than a Kuiu or MR. I like MR, but they're heavy and require an added upgrade to the frame to achieve any lift above the shoulders. A Kuiu Icon is one of the last packs I'd ever spend money on.
I'd take a pretty hard look at a lightly used Kifaru or Stone Glacier.
Tanner
Sound advice! Rockslide has used ones all the time under your price!
