In writing a job posting/ad, better to be specific or broad? In writing a job posting/ad for a tenure-track faculty position, is it better to (A) specify subfields the department is most interested in hiring in or (B) make the posting as broad as possible (even if there are some specific subfields the department prefers). 

Obviously, better is subjective here. Here are some more specific subquestions. I am open to making this community wiki if it's still not specific enough. 

Does (A) or (B) result in stronger candidates for the specific subfields? 

Does (B) actually discourage some strong candidates from applying?

Does (A) end up producing a weaker sequence of hires in the long term? 

I'm interested in this in terms of benefit to the department doing the hiring, not benefit for the candidates. 

Anecdotal evidence based on substantial experience is fine. Even better is research on the subject.
There is no straightforward answer to this. I would say it should depend on how important it is that the candidate fit the desired specialism(s). If you are willing to consider an outstanding candidate who does not fit the desired specialism(s), you should tend towards making the advert relatively open. If not, you should be more specific. In all cases, you advert should be open enough not to discourage serious contenders but specific enough not to waste the time of candidates who stand absolutely no chance. When advertising, it is also useful to clarify:


the history and modus operandi of any particular initiative/project/grant to which the position is connected (each institution and department has its own character and traditions, which may be obvious to an insider but may seem bizarre/confusing to an outsider); and
whether the purpose of the position being advertised is to


deepen an existing departmental strength or research cluster, or
broaden your department to extend coverage beyond its existing specialism(s).



Issues to consider with option (A)


it has the advantage of encouraging candidates with the corresponding specialism(s), who may otherwise not apply because they do not realise you are after those specialism(s) (because applying for jobs is very time-consuming);
it may discourage candidates whose profile is not a perfect fit (because they will assume that there must be someone who is a better fit in terms of specialism(s));
too specific, and people may assume that the vacancy is "fake" because you have already decided whom you intend to hire and are advertising to hide the nepotism (and thus not bother applying).


[but not all very specific adverts are stitch-ups: my current position had a very specific advert, but that was because it was to fill a post for which project-specific funding was already obtained; despite not knowing any of the people in any of the departments involved, I got the job, but I suspect I may have been the only applicant (the timing of the advert was unusual)]

Issues to consider with option (B)


you will probably get a lot more applications (no comment on whether this is a good or a bad thing); but
many/most of these applications will not fit your desired specialism(s); and
some candidates who would be a perfect fit, not being aware of the fact, may not bother applying;
people may assume that the vacancy is "fake" because you do not really intend to hire anyone, but are just "testing the waters".