Onkyo TX-NR646 vs Onkyo TX-NR656 - which one is the best
I have studied user reviews and checked different forums as well as official websites to provide you with accurate information that will help you choose the best model for you. Both models are great, but there are still some differences that should not be ignored. I want to pay attention to the most important features of these two models.

Onkyo TX-NR646 – a perfect device with many functions. What else do you need?
The NR646 will be able to play DTS:X besides Dolby Atmos via a firmware update available "soon". No such update is planned for the NR636. What this means is, the TX-NR636 will give you full Dolby Atmos multidimensional sound from Atmos-enabled discs (as well as True HD/lossy Dolby tracks) , but will produce only DTS-HDMA 7.1 tracks (and its DTS lossy variants) from DTS:X discs.
Movies won't ALWAYS use Atmos, they may use DTS:X, too. And X may even become the dominant multi-dimensional audio format for the next few years. But if you don't care at all for multi-dimensional audio/ceiling & reflective speakers and stuff, there are some other great amps on this same price range that don't feature Atmos/X but compensate on power/other features to give you a better bang for your buck.
- Supports the most part of modern audio formats. You can listen to your favorite music tracks without any problems.
- The total power output is 580 Watt. That is powerful enough.
- This model can use Bluetooth, network or USB devices as a source of content.
- It supports wireless interfaces (Wi-Fi, Bluetooth, etc.).
- It has different types of connectors. Including such types as TOSLINK, Mini-phone 3.5 mm,. phone stereo 6.3 mm, banana/spade, RCA, etc. some of the types are used rare and it is not a considerable advantage, but nevertheless, if you want to use some old devices then you will be able to do it without any troubles.
- Does not support the AIFF audio format.
- The model does not have such decoders as DTS neural:X, DTS-HD master audio and DTS:X. Therefore, you may have some troubles with playing some files.
- It does not have many additional functions.
- It has not digital content protection.
- 6 digital sound processor presets. It may be not enough for some customers.
Onkyo TX-NR656 7.2 Channel Network A/V Receiver
Great sound, very clean, supports 4k 60fps,HDR 10, PlayStation 4 and PRO compatible, Atmos and DTS X for object-based surround (3D audio), more than enough power for a middle to large rooms, two wireless antenna, wireless AC blazing fast wireless speed and supports multiple digital radio stations like: Spotify, Sirius Radio etc.
Easy to follow and read a menu, very straightforward not overly complex. Has two menu settings 'quick menu' for basic controls: subwoofer levels, on and off calibration settings. The more compact remote which supports fast sound mode switching for movies, games and music. Keep pressing the button it cycles through various built-in modes.Still getting firmware updates: latest one was Feb 23 which added: Support for FireConnect and enabled streaming from Network.
- This model supports almost all modern audio formats, like FLAC, MP3 or even Apple lossless.
- Powerful decoders are able to play almost any kind of files without any problems.
- The total power output is 1190 Watts. That is more than enough for you to enjoy your favorite music. It is super powerful.
- The device can use such wireless interfaces as Wi-Fi, Bluetooth, IEEE 802.3.
- This model can use a network, Bluetooth and USB devices as a source of content.
- It does not support the AIFF audio format.
- It does not have a decoder to decode DTS neural:X.
- This model does not have such connector types as 4 pin USB type A, 9 pin D-Sub, RCA.
Onkyo TX-NR646 vs Onkyo TX-NR656 - What is the best option for you?
These two models have similar names but they are completely different. They are able to play the most audio formats, they have powerful decoders. However, Onkyo TX-NR656 is more powerful and it is more advanced as the next version of a device. It does not have some connector types and that is not a big deal, especially if you do not have devices that use those kinds of connectors.
If you need a modern super powerful device, then Onkyo TX-NR656 is for you. If you have some old devices and the power output is not that important for you, then consider buying Onkyo TX-NR646.