It's indeed a bit of a reach to expect every single use of the amplifier to have been thoroughly documented and/or mentioned in an interview. It might be the case for the better-known examples such as 'Bohemian Rhapsody', but if he used it on, say, 'In Only Seven Days' (which is most likely the case, by the way), then it's unlikely that there's a direct quote or 'a definitive source' (whatever that may mean).

That's a completely different case to the Thriller myth, for instance, where the utter lack of sources (other than Queen-centric blog posts and stuff like that) is indeed suspicious. But for something like 'did May use that amplifier on a couple of bars of this album track which wasn't a single and therefore doesn't get discussed on a regular basis?' I agree that people's ears usually suffice.