preciarse is a very proud thing, so it is a bit questionable to do it from a moral standpoint; or at least it is tasteless . Subjunctive turns such characterization uncertain, killing any pretentiousness in it (not all of them but just those ones who
se precian)
Un melómano que se precie de serlo
Un verdadero melómano
un melómano que se
precia de serlo
because being -improperly- proud of it is not a part of the definition.