I frequent another forum (SAAB) where a member said that KYB stood for “Keep Your Bilsteins”!
I have replaced the rears on my 9-5 with B-6’s as one of the OEM shocks was leaking at 100000 miles and it has made a tremendous difference in ride quality and handling. On that car i do have upgraded sway bars as well which made a big difference as well. Fronts are on order through Summit but the lead time for B-6’s in my application is pretty long so a bit frustrating. I’m sure it would be better for a Subaru.
I am in Michigan and the roads there are horrendous so if your after a soft ride the B-4’s are probably better but if your after handling get the B-6’s. My wife’s Forester will remain stock because she couldn’t care any less but from my experience the Bilsteins are worth every penny!
YMMV but i think you’d be happy