It makes sense comparing Denon AVR-S640H vs Marantz SR6014 because they both are able to give some premium features.
A significant difference between the receivers in the number of channels, Denon AVR-S640H has 5.2 versus 9.2 for Marantz SR6014. Regarding power, then AVR-S640H has such a W/Ohm ratio - 75/8, 100/6 when SR6014 has a power of 110/8, 185/6. The THD is the same and is 0.08%.
Characteristics of digital to analog converter (DAC) are different, 192 KHz/32-bit for AVR-S640H and AK4458 384 KHz/32-bit for the SR6014. Only the SR6014 supports Bi-amping feature. Each of the AV receivers 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 SR6014 can receive a content from Deezer, TIDAL, Pandora, TuneIn Radio, iHeart Radio. Compared AV receivers retain the quality of 4K/60Hz signal when transmitting from a source to a TV or projector. HDMI signal transmission in standby mode is implemented in each of the devices. The SR6014 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 8/3 HDMI connectors of the Marantz SR6014. The HDMI Audio Return Channel (ARC) feature supports both devices. The HDMI eARC is available only on the SR6014. 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 SR6014. 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 SR6014 can be configured using the Setup assistant.
Only the SR6014 has a Dolby Atmos multichannel audio format support. A surround technology DTS:X is supported only by the SR6014.