The RPI4 has eliminated the issues sometimes seen with RPI3 running Ethernet and usb together. But you might consider the FLIRC heatsink case - it looks quite neat and keeps the RPI4 cool. I would be surprised if you got any USB noise feeding the CXA81 directly from the RPI4. To look for it play a very quiet track (before the recording gets going). Play at max volume and listen for any mid/hi frequency whistles/tones. If you can’t hear them with your ear near the speaker then they aren’t there!
You can certainly try Archimago’s clock adjustments to run things cooler and possibly reduce noise in some setups, but if you’re not experiencing an issues it probably isn’t necessary.
To better the DAC in the CXA81 you would need to consider something very high end like the Topping D90 or the OktoResearch dac 8 stereo - but be warned you are very much in the area of diminishing returns and are unlikely to hear any improvements!
So try the low cost solution with the RPI4 and standard USB cables
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