It makes sense comparing Onkyo TX-RZ840 vs Onkyo TX-8020 because they both are able to give some premium features.
A significant difference between the receivers in the number of channels, Onkyo TX-RZ840 has 9.2 versus 2.0 for Onkyo TX-8020. Regarding power, then TX-RZ840 has such a W/Ohm ratio - 180/6 when TX-8020 has a power of 90/6. The THD is 0.08% for the TX-RZ840 but 0.03% for the TX-8020.
Characteristics of digital to analog converter (DAC) are different, AKM 384 KHz/32-bit for TX-RZ840 and 192 KHz/24-bit for the TX-8020. Only the TX-RZ840 supports Bi-amping feature. Each of the AV receivers can transmit an audio signal directly to the amplifier and bypasses any DSP processing.
Only the Onkyo TX-RZ840 can connect to the Internet via WI-FI. Only the TX-RZ840 has Bluetooth support. The Onkyo TX-RZ840 has support for AirPlay, AirPlay 2, and its competitor in our comparison - AirPlay. Only the TX-RZ840 supports Spotify. Only the TX-RZ840 provides HDMI signal transmission in standby mode. Both competitors are not able to scale the HDMI signal. Dolby Vision technology found support only on the TX-RZ840. Only the TX-RZ840 supports HDMI Audio Return Channel (ARC). The HDMI eARC is available only on the TX-RZ840. Of the two receivers in our comparison, HDMI CEC is present in only the TX-RZ840. These compared receivers have a built-in phono stage for connecting a vinyl player. An ECO mode is not present in each of the AV receivers. The setup assistant will help you configure Onkyo TX-RZ840.
Only the TX-RZ840 has a Dolby Atmos multichannel audio format support. A surround technology DTS:X is supported only by the TX-RZ840.