I owned the X1600, x3700h, Marantz sr6015, all sounded really bad compared with Yamaha V6A. After more than 30 hours of Audissey calibration, never managed to make it sound good. Human voices sounded unnatural, muffled, explosions were lacking dynamic, back speakers were overpowered. Upmixer sent voices in the back speakers or played them lauder through the LR speakers than the center. The music especially sounded really dull. I tested the Sound United AVR with three brands of speakers, Focal, QAcoustics and Kef. I was amazed how bad all speakers sounded despite the great reviews. I realized that it was not the speakers. It was the avr. In the end I returned every Denon and Marantz and bought the Yamaha V6A. From the first calibration it sounded much better than the more expensive Denon and Marantz.
For me HDMI 2.1 means nothing if the sound is bad.