It makes sense comparing Marantz SR6015 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, Marantz SR6015 has 9.2 versus 2.0 for Onkyo TX-8020. Regarding power, then SR6015 has such a W/Ohm ratio - 110/8, 150/6 when TX-8020 has a power of 90/6. The THD is 0.08% for the SR6015 but 0.03% for the TX-8020. Only the SR6015 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 SR6015 has Bluetooth support. The Marantz SR6015 has support for AirPlay, AirPlay 2, and its competitor in our comparison - AirPlay. Only the SR6015 supports Spotify. Only the SR6015 provides HDMI signal transmission in standby mode. The SR6015 can scale the input HDMI signal, unlike the TX-8020. Dolby Vision technology found support only on the SR6015. Only the SR6015 supports HDMI Audio Return Channel (ARC). The HDMI eARC is available only on the SR6015. Of the two receivers in our comparison, HDMI CEC is present in only the SR6015. These compared receivers have a built-in phono stage for connecting a vinyl player. An ECO mode is only available for the SR6015. The setup assistant will help you configure Marantz SR6015.
Only the SR6015 has a Dolby Atmos multichannel audio format support. A surround technology DTS:X is supported only by the SR6015.