I cannot conceive going back to a receiver if you have had quality separates. Too many compromises are made in the building of a receiver. Besides, it is easier to upgrade when you only have to upgrade one thing at a time (separates) rather than having to to it all at once (a receiver).
You may wish to reconsider your thinking. There are always exceptions.
Hawk is right in my opinion. While Denon, Yamaha, Marantz and other companies may make some very nice receivers, if you are after a great two channel system, you would be wise to forgo the receiver route (and probably the integrated amp, as well). I think this is particularly true if you are looking at the kind of budget most top end Denon or Yamaha receivers command.
Personally, for $4k, you could get into an extremely nice Cary tube amp (like the V12, or Six-Pacs or similar) and a nice pre-amp from the likes of Van Alstine, Rogue Audio, etc.,. Any of those combinations will blow away the Denon. But, if you are really big into Home Theatre and you are willing to compromise two channel listening, a receiver or integrated may be the way to go.
And personally, I would much rather have a nice integrated from Jolida or Antique Sound Labs - at half the Denon's price and twice its performance!