I don't have any experience with either unit, but I can give my thoughts based on what I know and have read.
The Parasound JC2 uses fully discrete FET analog stages. I suspect that it is going to give more transparency and resolution than the Krell Foundation. It's an excellent sounding preamp. The one caveat on this is that it's been written that this JC2 preamp has slightly weak midbass.
The Krell Foundation has excellent power supply capacitor banks and will not have the bass/midbass problems. However, it does use op amps for all analog stages. If you are looking for more bass/midbass punch and are willing to sacrifice some elements of resolution/transparency, then the Krell could be a good choice.
Both units sound very excellent and would be significant upgrade over your Denon AVR. Thinking about your music, if most of it is 80's pop and metal, then the Krell Foundation might be the right choice because the JC2 could have a tendency to sound too thin/bright/harsh with the metal. If you preferred stuff like jazz/classical, then the JC2 would probably be the right choice because it would reveal more of the "realness" of jazz vocal and instruments.
In either case, you would have to buy a separate phono preamp because neither the JC2 nor the Krell Foundation have phono inputs. I did notice that your Denon 4520ci does have phono inputs, which is rare for an AVR.
There are other HT processors that are good. Classe SSP-800 is excellent, but also uses op amps (not sure if it's actually better than Foundation). One of the things I like about Krell is that they DO NOT use switching power supplies in their processors. Everything is done from linear power supplies, including digital sections. Even the Classe uses switching power supply for digital sections.