It seems like you see refurbished and think trouble? The Denon 4500 is factory refurbished. That's a big difference from a regular return. It might have never been used or it might be a return or a demo. BUT all of them are fully tested. For me it's a waste of money buying new and paying more. These units come with a 3 year warranty. That's amazing on a refurb.
The Denon 4500 is able to individually set the distance and levels for each sub independently. You can later change the levels independently or as a group keeping the volume ratio the same.
I had a Yamaha receiver (lower end). It died on me right after the warranty ended. I have three Denon receivers and two are really old and never die. My Onkyo left channel stopped working, My Yamaha won't even turn on but my Denon's keep running and running. Everyone's experience is different. You might need to pay extra for the peace of mind placebo?
I worked for a company refurbishing and testing product. I would rather have a unit that's been individually tested at the factory than a new unit. My behind the scene experience may be different than yours.
Buying a return from a store that doesn't have all the parts to make a unit the same as new is a different story and these idiots on eBay that say owner refurbished on their description scare me.