I know what you mean. Those claims are mostly from Marantz owners. D&M will tell you the same because they frequent those forums and therefore leant to tell people what they like to hear.
I happen to have owned 4 Denon AVRs, two Marantz prepros, one Marantz preamp and two Marantz power amps in the past 8 to 9 years, and I can tell you without reservation that even my cheapest Denon don't sound bright and my most expensive Marantz separates don't sound warm. In my setups, bright, warm, or anything in between is a function of the quality of the source media.
Since you seem to have formed a bias towards believing Marantz must sound better for music, regardless that is baseless, hearsay, or factual, you really should go for the Marantz. Their prices are actually quite comparable as long as you compare the X4000 with the SR6008 and the 4520 with the SR7008. The Denon may even costs a little more.
If you go with Denon now, you may end up wondering if you should have gotten a Marantz, everytime you feel something don't sound right regardless of the real root cause. Why would you even consider Denon anyway, is it just for the XT32?