Assume that point depends on users ears, but what it the generally accepted number beyond which it has no value? (or range)
This is just me, but I broadly interpret "good enough to not matter" to be around 105dB SINAD for typical 16 bit content. My thinking is that since noise/distortion is additive, and you'll always have a digital->analog step and an amplification step, you want to ensure that the resultant THD+N from the entire chain isn't appreciably adding to the noise already coming off the source material (theoretical maximum dynamic range for dithered 16 bit content is 93dB). Assuming two steps, a SINAD of 105dB at each step results in a theoretical reduction of ~0.5dB. Going above 105 doesn't bring much benefit; however, dropping below 105dB, things get ugly in a hurry.
Example: if both your DAC and your amp have a SINAD of 93dB (to match that of the theoretical DR of Redbook), the resultant SINAD of a system playing back 16 bit content is ~88dB. Assuming both components have 105dB SINAD, you're looking at ~92.5dB. Going the other direction, if both of your components are SOTA (117dB SINAD), the output will have a SINAD of 92.98dB.