I'm 5'9" and have both a 52cm 2008 Jake and a 54cm 2008 Major Jake (same geometry as the 2010 JtS). The 54 fits me perfectly, but with the right stem I can get the same fit out of the 52. Likewise, I'd imagine that with a stem swap you could get your 56 to fit more or less like a 54. If standover height isn't a problem, I would guess you can dial it in pretty well. Of course, you've got the bike already so you can figure out whether or not you are happy with the size. How it feels to you is really the only thing that matters.
I agree, BTW, that you would have to do a lot of shopping to find a better-or-equal replacement for $750.