his dudeness said:
they share a lot of components but different design,marantz use there hdam moduls from there stereo amps as well which i think greatly improves the sound.
For what is worth, my experience is just the opposite. I own units of both brands and became brand-agnostic, but certainly not price agnostic.
I ordered from a large Canadian retailer which allows easy 30-days returns a Marantz 7008 and a Denon x4000: in a month of back and forth test I could not tell a difference and this was with a 4000$ speaker setup.
A musician fiend of mine could not either and he has very good ears of a piano player since he was 6 y/o (now 43). Kept the Denon which costed a hefty 600$ less.
Another clue: all the firmware updates are posted the same days on Denon and Marantz web sites and are likely to be the same. This should confirm they share the same internal components, while it does not resolve the doubt of the different tuning but my ears did: no difference.
My conclusion is that the difference now is only in the marketing in some of their offerings, althought it has to be said that Marantz offers the slimline which is missing from Denon (for now). I also own the Marantz NR-1504 and I guess if a cheaper comparable receiver was available from Denon I would have gone for the second Denon.