It makes sense comparing Onkyo TX-NR555 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-NR555 has 7.2 versus 2.0 for Onkyo TX-8020. Regarding power, then TX-NR555 has such a W/Ohm ratio - 130/6 when TX-8020 has a power of 90/6. The THD is 0.08% for the TX-NR555 but 0.03% for the TX-8020.
Characteristics of digital to analog converter (DAC) are different, Hi-Grade 384 KHz/32-bit for TX-NR555 and 192 KHz/24-bit for the TX-8020. Only the TX-NR555 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 TX-NR555 has Bluetooth support. The Apple Music service is implemented on considered devices: AirPlay. Only the TX-NR555 supports Spotify.
Only the Onkyo TX-NR555 has support for High dynamic range (HDR). Only the TX-NR555 provides HDMI signal transmission in standby mode. Both competitors are not able to scale the HDMI signal. The HDR standard - Dolby Vision is not supported by these receivers. Only the TX-NR555 supports HDMI Audio Return Channel (ARC). Unfortunately, HDMI eARC is not available on monitored devices. Of the two receivers in our comparison, HDMI CEC is present in only the TX-NR555. These compared receivers have a built-in phono stage for connecting a vinyl player. A voice control is not available. An ECO mode is only available for the TX-NR555. Unfortunately, the setup assistant is not available in each of the models.
Only the TX-NR555 has a Dolby Atmos multichannel audio format support. A surround technology DTS:X is supported only by the TX-NR555.