I remember reading about the development of a Call of Duty game where during multiplayer testing they gave all the guns the same statistics.  Everybody thought the AK47 had more power and recoil than the M4A1, which was more accurate but less powerful.   Obviously this is anecdotal evidence but it sums up a lot of people's own bias.  Price, size, aesthetics, past experiences, etc. all play a role in how we determine our opinions.  I still believe if Schiit put the Vali in a Mjolnir case and charged $999 for it people would think it was much better than in the little magni/modi case and a $119 price tag.

I think that the more educated and experienced you are in reviewing, the better the reviews will be.  The Vali improved the bass quality of my setup over the Anedio D2 headphone output. I could accurately say that the Vali has the best bass out of any amplifier I have heard.   Purrin has clearly stated that he thought the Vali had one note bass.  Neither of us is wrong, though I would give his impressions far more value than mine because he has more experience to help him draw conclusions.  While this isn't necessarily bias or neutrality, I think it is an aspect of reviews that can't be separate from the aforementioned problems.  From my review it is impossible to say whether I liked the Vali's bass because I like Schiit, or because I like tubes, or because my lack of experience led me to a different conclusion.  There is no way to get around that fact and merely stating that I am being completely neutral and honest can't do anything to prove that to readers.

The best way I can think of to validate these reviews for yourself is to independently arrive at similar conclusions on a given product and to extrapolate from that your own conclusion on honesty and neutrality.  Unfortunately we often use the reviews to determine if we want to buy a product, which will inherently bias our impressions of them.

In the end I try not to worry about it, just accept that all reviewers try to validate themselves to readers by stating that they are neutral and unbiased.  The reality is that most people are unable to understand why there is no such thing as neutrality anyway.