A couple of thoughts...
Is there a possibility that your MRX 500 can be repaired for a price you consider reasonable? You mentioned that you like the sound, making a repair potentially your least expensive solution. Of course, putting money into a 6 year old AVR in some ways is like putting money into an old car: it buys you time but it's gonna break again some day. And if it lacks features you've been wanting, a repair may be even less interesting.
Assuming you go down the replacement route, there are quite a few AVRs at or below the $1399 MSRP of the Anthem MRX 520, each with different feature sets and different sounds. You may need to go listen to units of interest to decide what you like. At point of purchase, be sure you understand the return policy in case it isn't quite what you expect. Getting a loaner from an AV dealer would be even better.
Do you use the room correction software? If so, I'm not aware of anything better than ARC at this price point. YPAO, Audyssey, Dirac and others all have their fans, and I have not listened to all of them as implemented in various AVRs. I can only say that in my room, with my gear, in my house, etc. I have found ARC to be an excellent solution.
Last thought, since it looks like you hold onto your gear for a while (me too
), it might be worth your time to think about capabilities you might want over the next few years... just in case. If the 520 still fits your needs, that could be a decision. If not, there may be some research ahead.