There are two camps: subjectivists and objectivists. The first listen with their ears and the latter make measurements.
Any scientifically minded person is liable to lean towards the objectivist camp. The O2 was designed by measurement. Essentially you know an amp should be good if the published specs say so *and* are adequate (some specs leave out essential details). After that you need a decent hifi mag or 3rd party to test and measure the amp. Then you really know.
There is a problem, however. Has every sonic characteristic been measured? Are all these measurements enough? Perhaps 70 years ago some one them were not measured. Perhaps there are unknown qualities to audio yet to be discovered. This plays straight in to the subjectivist camp and is unanswerable.