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.
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-