Only just read this thread. To answer your question, as least as far as Stereophile is concerned, the fundamental requirement for an audio reviewer is the same as for any writer: to have something to say and be able to say it. I have slowly built up my reviewing team over the years based on these requirements as they apply to audio.
These requirements may sound simple, but it is actually rare to find someone who meets them. Hardly any of the people who approach me about reviewing for Stereophile get even close.
And regarding the subject of this thread, how much a reviewer gets paid is between him and the IRS, surely.
John Atkinson
Editor, Stereophile