I do appreciate your feedback. After much consideration, I have amended my order to the Corsair 4000D Airflow case, the Corsair 550W TXm Series PSU and this combination would only have cost an extra £25 extra as I have now ditched the ASUS Xonar SE card, but unfortunately that brought up a message informing me that my selected video card was in short supply, so I have reluctantly had to “upgrade” to the Nvidia GeForce GTX 1660 which means I have now paid £99. Shame that was the only choice available. I think you’re right in that I’m sure there shouldn’t be any discernible difference with the TUF B550 handling the sound. Bearing in mind that I am replacing a 10-year old Intel i7 860 @ 2.80GHz with an AMD Radeon HD 5850 and ASUS Xonar sound card, I’m sure that things will have improved somewhat since then.It's about (a) longevity, (b) efficiency, and (c) quietness, as well as reliability. Don't cheap out on power supplies. Will it work? Sure. But a better PSU is a better choice. Your machine, of course.
Go for the 4000D Airflow, then. It's only £10 more than the Focus G and far better than the cheaper cases. The openness allows airflow, which means cooling and therefore quieter operation (important if you're interested in audio stuff). Will you get more dust build-up? Maybe though that's a complex question, and the mesh filters Corsair make are excellent, while cleaning it out is easy. A cheaper case with poor airflow means cheaper fans that run at a higher speed all the time, and therefore make extra noise.
That's only a tiny bit better than what comes included on the Tuf B550 (192KHz, 24-bit, 108dB SNR). If you're really interested in audio quality, get an interface like the Focusrite 2i2.