The Denon AVR-X8500H vs Denon AVR-S660H comparison proves that these receivers have many common features.
A significant difference between the receivers in the number of channels, Denon AVR-X8500H has 13.2 versus 5.2 for Denon AVR-S660H. Regarding power, then AVR-X8500H has such a W/Ohm ratio - 150/8, 250/6 when AVR-S660H has a power of 75/8, 100/6. The THD is 0.05% for the AVR-X8500H but 0.08% for the AVR-S660H.
Characteristics of digital to analog converter (DAC) are different, AK4490EQ 768 KHz/32-bit for AVR-X8500H and 192 KHz/32-bit for the AVR-S660H. Only the AVR-X8500H supports Bi-amping feature. The Denon AVR-X8500H can transmit an audio signal directly to the amplifier and bypasses any DSP processing. Rivals from our review have Bluetooth support. The Apple Music service is implemented on considered devices: AirPlay, AirPlay 2. Spotify can be used on each receiver. The AVR-X8500H can work with the audio stream from Deezer, TIDAL, Pandora, Napster, SiriusXM, TuneIn Radio, iHeart Radio, Sound Cloud, and the AVR-S660H can receive a content from Deezer, TIDAL, Pandora, SiriusXM, Napster, SoundCloud. HDMI signal transmission in standby mode is implemented in each of the devices. Both rivals can scale the resolution of the incoming HDMI signal. The HDR standard - Dolby Vision is supported by these receivers.
The Denon AVR-X8500H has 8/3 HDMI inputs/outputs versus 6/1 HDMI connectors of the Denon AVR-S660H. The HDMI Audio Return Channel (ARC) feature supports both devices. HDMI eARC is available in each model. The Consumer Electronics Control (CEC) feature is present in most modern AV receivers and the models in our comparison are no exception. The AVR-X8500H supports the protection of digital content of the standard HDCP 2.2, versus HDCP 2.3 in the AVR-S660H. These compared receivers have a built-in phono stage for connecting a vinyl player. Supported voice control technologies are identical - Amazon Alexa, Google Assistant, Siri, Josh.AI. It is also worth noting that the ECO mode is presented in each of the receivers. Both AVR-X8500H and AVR-S660H can be configured using the Setup assistant.
Both devices have support for Dolby Atmos multichannel audio format. The surround sound technology DTS:X is supported by devices under review.