What should be mandatory in every professional published review-

Required? 

Nothing, other than they should really listen to the piece of equipment other than toss the gauges on it.  You know like that one website. 

If the word "mandatory" is freaking people out how about each publication developing its own "template" that their stable of reviewers would adhere to in order to insure some continuity and comprehensiveness to formally reviewed products. If I were the editor of such a magazine or online publication, I would suggest the following:

1. Room dimensions along with any room treatments;
2. Associated equipment;
3. Statement of preferences--as an audiophile, what am I shooting for in terms of sonics;
4. Relationship with manufacturer--any financial interest, discount offered on purchases, history with brand, etc.

Seems to me this would help the buying public make better sense of the reviewers findings.

would be nice, but I agree, completely impractical/impossible to always have the previous unit on hand for comparison...and then, there are so many other products it would be great to compare it to...not just the XP20 but an ARC, a CJ, JRDG etc. 

I would place the responsibility on the manufacturer to explain/demonstrate to their clients what is different between the former version and an upgraded or updated version.

I place the responsibility on the reviewer to,

1. At a minimum, use partnering equipment that would reasonably be expected to be used with the review equipment - i.e., no mis-matches, and to
2. Compare the reviewed equipment with comparable competitors available in the market at the time of the review.

I was fortunate to have seen reviews by the same reviewer, of my new DAC and of the same manufacturer’s former DAC. The direct comparisons were certainly helpful in knowing what I should expect. However, that will mostly not be practical, as several here have pointed out. 6moons usually does a good job of making meaningful comparisons, IMO.

What makes someone a professional reviewer ?