Having had an opportunity to test Denon AVR-S640H vs Yamaha RX-A880, I’d like to share my impression here to facilitate your choice of a new AV receiver.
The number of channels for receivers is different - 5.2 versus 7.2, respectively, for devices. In terms of power, there are differences, the AVR-S640H has such a ratio W/Ohm - 75/8, 100/6, while the RX-A880 has a power of 100/8, 160/4. The value of total harmonic distortion is also different, and for the AVR-S640H it is 0.08%, but for the RX-A880 it is 0.06%.
The manufacturer installed different DACs, the AVR-S640H received the model 192 KHz/32-bit, but the RX-A880 is equipped with Burr-Brown 384 KHz/32-bit. The Bi-amping function is implemented only in the Yamaha RX-A880. Both Denon AVR-S640H and Yamaha RX-A880 can transmit an audio signal directly to the amplifier and bypasses any DSP processing. Each of the receivers has a Bluetooth connector. Both devices can work with such Apple Music services: AirPlay, AirPlay 2. Access to the Spotify service is on each of the competitors. The number of available streaming services varies for receivers, the Denon AVR-S640H works with Deezer, TIDAL, Pandora, Napster, SiriusXM, TuneIn Radio, iHeart Radio, Sound Cloud, but the Yamaha RX-A880 works with Deezer, TIDAL, Pandora, Napster, SiriusXM, Qobuz. The quality of 4K/60Hz signal remains unchanged when transmitting from a source to a TV or projector via HDMI for both receivers. Signaling over HDMI in standby mode can provide the AVR-S640H and the RX-A880. Video scaling is only possible with the RX-A880. Each of the participants of the comparison has a Dolby Vision.
The number of HDMI inputs/outputs varies depending on the receiver, the AVR-S640H has 5/1, and the RX-A880 has 7/2. HDMI Audio Return Channel is implemented in both devices. HDMI eARC is only supported by the RX-A880. When comparing receivers, it becomes clear that each of the rivals can work with HDMI CEC. AVR-S640H and RX-A880 support the standard HDCP 2.2. Like most receivers, our models have a built-in phono stage for connecting a vinyl player. For voice control in the AVR-S640H is used Amazon Alexa, Google Assistant, Siri, Josh.AI, but in the RX-A880 is used Amazon Alexa, Google Assistant. There is an ECO mode in either AV receiver. The Denon AVR-S640H and the Yamaha RX-A880 can be properly configured using the setup assistant.
A Dolby Atmos multichannel audio format is available only in the RX-A880. Only in the RX-A880 realized a DTS:X surround technology.