Both units are now ESS DACs. Older AVM70 are AKM. They had released (and removed) a spec sheet for the AVM90 in early Feb that listed the ESS DAC. I was also able to identify a different ESS DAC for the newer AVM70s via photo of the guts.
AVM90 vs. AVM70
1. DACs are the very best made from ESS, 16 times the cost of DACs of AVM70
2. Has updated circuitry (higher end for better sonic performance)
3. Supports 4 independent subs vs. 2
4. Comes with HDMI 2.1 fully supported vs. upgrade path...