Interesting thread.
Those folks who have the “immediate predecessor” product might be able to glean certain insights from a new product review. That may come from a variety of sources, such as the reviewer’s personal biases in reproduced sound (and whether they resonate with those of the reader) as well as prior reviews. For example, I like a lot of what PHD of TAS reviewed because I have similar biases to his own. It’s just a flavor for what might be of interest to me if I were in the market for that particular item covered by the reviewer. I suspect reviewers develop a faithful following based on the contents of their reviews, regardless of product comparisons they might make. So if the OP demands certain requirements of the reviewers that he follows, then that’s his preference. But every reviewer has a different audience who might differ in what they want from a given product review and reviewer. Mandatory requirements? Maybe for certain audiences yes, but by no means not all audiences.
What should be mandatory in every professional published review-
When testing a company's newest amp, preamp, etc, and it is a refinement of a prior product that was on the market, ie, a Mark II, an SE version, a .2 etc, it should be mandatory that the review includes a direct comparison with the immediate predecessor. IMHO, it's not enough to know ion the product is good; it's also important to know if there is a meaningful difference with the immediate predecessor.
I'm fan of Pass Labs, and I just looked at a review of an XP22 preamp. I find it very disturbing that there was no direct comparison between the XP22 and the XP20. And this lack of direct comparison is ubiquitous in hi-end published reviews, across all brands of gear tested. I don't blame the gear manufacturers, but rather the publications as I view this as an abdication of journalistic integrity.
Opinions welcome-