The Denon x4400h is still available on clearance at Acoustic Sound Design and Sound Distributors, but probably not for much longer, looks like the other online authorized dealers are sold out. Crutchfield (and probably other places) has the Klipsch RP280f on clearance at a very attractive price. Not saying they're the best, but I like them, and they reviewed well. That's about $1800, spend the rest on subwoofers or more speakers.
The Denon can be expanded to 7.2.4 atmos, so you're covered there. Since you're more music than theater, I'd probably look hard at Rythmik and Monoprice for subwoofers. The matching center is also on sale for the same price (RP-450c), as are the rp160m for slightly less, which you could use as surrounds, and still leave over $1000 for a subwoofer.
And, yes, I use the RP series with Rythmiks, and I love the combo for music.
So, for your budget, you can get the X4400h, the front 3 speakers, and a Rythmik FV15HP, and a little over, add RP160m for surround, you have a quality music/movies setup for about $4000, and all you need to add later is an inexpensive stereo amp, 4 Atmos speakers, and there you are.