To me, the newer crops of avr/avc do decode better, assuming you meant the DSP (hardware and software) functionality.
As for the ICs, Denon/Marantz has updated the volume control and multiplexor/switche ICs for their whole line up since 2017 for sure but likely 2016, along with the DAC.
The DAC chip in the X6400H (same for the X1400H and SR5012) has much lower distortions+noise, 0.00045% vs 0.002% (lower better obviously), or in SINAD, 107 dB vs 94 dB (higher better) than the one in the X5200W.
The volume control chip in the X6400H (same for the X1400H and SR5012) also has better THD+N specs and the switches, being separate from the otherwise part of a LSI (large scale integrated) chip as well. For more information, read the introduction section of the linked article by Dr. Rich
Good News! Marantz Says No More AVR LSIs for You! You might recall my article on Audio Video Receiver Build Quality announcing the downgrade of the analog section of almost every AVR and Pre Pro coming from the major Far Eastern manufacturers. The article discussed how multi-channel audio...
hometheaterhifi.com
As HD alluded to, while specs do support the newer D&M product are much better, it is not clear how many, if any at all, can tell the difference between the X5200H and X6400H in a properly conducted listening tests when the effects of bias are mitigated.
For example, is the difference between the 0.002% THD+N for the X5200W's DAC and the 0.00045% for the X6400H's audible? I have seen some Krell FPB amps that didn't measure all (in terms of THD+N)that well vs AVRs, definitely much worse than even the DAC in the AVR-X5200W, yet I bet you think it improves SQ for you even at low listening level right?