Denon AVR-S660H vs Denon AVR-S670H comparison definitely makes sense.
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-S670H can transmit an audio signal directly to the amplifier and bypasses any DSP processing.
WI-FI modules have the same characteristics: 2.4/5GHz. 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-S660H can work with the audio stream from Deezer, TIDAL, Pandora, SiriusXM, Napster, SoundCloud, and the AVR-S670H can receive a content from Deezer, TIDAL, Pandora, SiriusXM, Napster, SoundCloud, Amazon Music, Amazon Music HD, MOOD:Mix.
Both AV receivers support HDR10+ technology (High dynamic range). Compared AV receivers retain the quality of 4K/120Hz signal when transmitting from a source to a TV or projector. 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.
Each receiver has 6/1 HDMI inputs/outputs. 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. Both models support the standard HDCP 2.3. 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-S660H and AVR-S670H can be configured using the Setup assistant.
None of the rivals equipped with Dolby Atmos multichannel audio decoder. The receivers do not support the surround sound technology DTS:X.