OK here's the answer again: tests are put together by our team of reviewers based on market awareness, their feeling for what will be of most interest to the greatest number of readers, their knowledge of interesting features which may make for meaningful comparisons and, as Clare has said, the availability of review samples. Yes, we have a limited budget to buy products for review, but we use that sparingly, and only when all the resources for loaning a sample have been exhausted.
No round-up is intended to be a comprehensive market overview, but rather a snapshot of some interesting products available at the moment, and as I have said, some models will get left out for whatever reason.
There's no manifesto for such tests, no peer-review of what's included, nor any statistical analysis of forum requests or anything else. The review team, with its extensive experience and market knowledge, simply puts together a bunch of products it thinks will interest the largest possible number of readers, even when it's a test of something as esoteric as DACs.
And let's face it, in a mass-market magazine as likely to be bought by someone buying a new TV or mini-system as a turntable, having a review of half a dozen DACs is taking a bit of a flyer, since most readers won't even have a clue what a DAC is, let alone ever consider buying one.