This is the new model that has been released with HDMI 2.1 on all of the inputs instead of just one, some new processing capabilities and a couple of new formats and future room correction add-on ability.
Reviews have not been as positive on this as the X3700h, but it is important to note that all current models you buy on clearance will have the same decreased noise performance that the new model is criticised for on Audio Science Reviews, and that the original models with the better chipsets had faulty HDMI 2.1.
I have been checking prices on this since launch and it has been pretty steady at RRP. 20% is a massive discount on such a new product.
The outgoing X3700h has also just tipped under $2k at other stores. I was going to buy that today, but at $400 difference instead of $1k more, I bought the new model using this to price match.
A bunch of their other Denon and new Marantz models also 20% off.