Complex sentence: Once subjectively valid methods have been shown to correlate well with listening tests over a wide range of conditions, then such methods become the preferred ones for all purposes, and it is important to realise that engineering methods are not even valid when comparing like with like.
Split into simpler sentences: Once subjectively valid methods have been shown to correlate well with listening tests over a wide range of conditions, then such methods are generally adopted as preferred. But it's important to realise that engineering methods are not always sufficient to when comparing like with like.