The stock BOV is actually quite audible, its just that it vents into the airbox so is muffled to near-silence. Stop it venting into the airbox and hey presto - a bit of swoosh-ing without off loading a couple of hundred sheets
Not as loud or \'funky\' (or gay - depending on yer school of thought!) than say a Blitz or HKS job but hey ho.
Meet up with a few people with variuos ones fitted and see what you like sound-wise the best - there isnt really any that are more functional than others, or the stock one come to that so its just a case of what you think sounds the most max power