Whether the change in sound quality was intentional or not is irrelevant. Naim must have known the firmware update changed the sound, it would be pretty amateurish if they didn’t monitor sound quality after a firmware update.
Plus we know sound quality variation is discussed amongst the beta testers. @HungryHalibut
What Naim should be doing, is, next the install update button, put a note.
Anyway, it appears Naim are not refusing to facilitate rolling back of the firmware. So, all good. 