The Denon AVR-S640H vs Denon AVR-S660H comparison proves that these receivers have many common features.
Considered AV receivers have the same number of channels 5.2. And as for power, it is the same and equal to 75/8, 100/6. The THD is the same and is 0.08%.
Competitors' digital to analog converter (DAC) is identical to 192 KHz/32-bit. None of the models support Bi-amping. The Denon AVR-S640H 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-S640H 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. The AVR-S660H can scale the input HDMI signal, unlike the AVR-S640H. The HDR standard - Dolby Vision is supported by these receivers.
The Denon AVR-S640H has 5/1 HDMI inputs/outputs versus 6/1 HDMI connectors of the Denon AVR-S660H. The HDMI Audio Return Channel (ARC) feature supports both devices. The HDMI eARC is available only on the AVR-S660H. The Consumer Electronics Control (CEC) feature is present in most modern AV receivers and the models in our comparison are no exception. The AVR-S640H 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-S640H and AVR-S660H can be configured using the Setup assistant.
Only the AVR-S660H has a Dolby Atmos multichannel audio format support. A surround technology DTS:X is supported only by the AVR-S660H.