There are some manufacturers who have been reviewed without buying advertising but I'd be willing to bet that they are few and far between.
I think there's a lot of different factors at work here...but I don't think there is a policy that ties advertising and reviews. Here are some perspectives that I have heard over the years:
- Some manufacturers have said "I will not send my product to Stereophile, because I do not advertise there and am afraid of receiving a bad review."
- Some editors have said "I will listen to every piece of gear that I receive for review, but if I really don't like it, I will simply not write a review."
- Some manufacturers have said "I will send in a product for review, and if they write a good one, I will advertise in their publication."
I have never heard an editor say "I will only give them a review (good or bad) if they advertise with us", "I will give a good review to them to encourage them to advertise with us", or "They advertise with us, so I will not publish this bad review".
So what reviews get published is impacted a little bit by both sides (reviewers and manufacturers).
Stereophile does have some other criteria for review (eg. how many dealers a manufacturer has, etc) that discourage them from reviewing products from smaller companies...