Having had an opportunity to test Denon AVR-S640H vs Pioneer Elite SC-LX904, 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 11.2, 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 Elite SC-LX904 has a power of 140/8. A common feature for devices is the same total harmonic distortion - 0.08%.
The manufacturer installed different DACs, the AVR-S640H received the model 192 KHz/32-bit, but the Elite SC-LX904 is equipped with ES9026PRO 384 KHz/32-bit. The Bi-amping function is implemented only in the Pioneer Elite SC-LX904. Both Denon AVR-S640H and Pioneer Elite SC-LX904 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 Pioneer Elite SC-LX904 works with Deezer, TIDAL, Pandora. 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 Elite SC-LX904. Video scaling is only possible with the Elite SC-LX904. 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 Elite SC-LX904 has 7/3. HDMI Audio Return Channel is implemented in both devices. HDMI eARC is only supported by the Elite SC-LX904. When comparing receivers, it becomes clear that each of the rivals can work with HDMI CEC. The AVR-S640H has support for the standard HDCP 2.2, compared to the Elite SC-LX904, which supports the standard HDCP 2.3. 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 Elite SC-LX904 is used Amazon Alexa. There is an ECO mode in AVR-S640H. The Denon AVR-S640H and the Pioneer Elite SC-LX904 can be properly configured using the setup assistant.
A Dolby Atmos multichannel audio format is available only in the Elite SC-LX904. Only in the Elite SC-LX904 realized a DTS:X surround technology.