denon 4300 can be had for about 700 or 800 these days, 6300 for around 1500. i have no complaints about my 4300, handles 4k, atmos/dts:x, just wish i had the extra 2 channels of the 6300
the x400 is current model, the only real upgrades (for me at least) are e-ARC, as i dont have a need for auro-3d. e-ARC itself is nice, but not worth the 2x price tag over last generation.
just remember you will want more. first i jsut wanted a better receiver for my klipsch 5.1 setup. then i said "well i should make sure it can handle atmos" - then it was "well 5.1.4 is fine" and now i have a 2nd amp added for 7.1.4 and im drooling over the new 8500 that can power 13 channels......