If you are considering a used JC5 then you should also consider a used CODA #8. It's just as good or better than the JC5 and has a transferable 5-year warranty. This amp has massive current reserves.
Will there be a dramatic difference?
I currently am running my Thiel CS 7 speakers on a 25 year old Carver amp I have an amazing Audible Illusions modulus phono preamp. It all sounds pretty incredible. I am considering Parasound a 21 and I’m wondering if that upgrade will make a dramatic difference in the way my system sounds overall? Would it definitely be worth the investment?