Three new Bluetooth headsets we’ve been testing generally follow the historic rule, but also interestingly diverge enough from it to be worthy of any iPhone owner’s attention. Two are from Plantronics—the brand new Voyager 520 ($100) is an improved sequel to the company’s popular mid-range Voyager 510, while the Discovery 665 ($150) is a smaller and lighter alternative at the top of the Plantronics lineup. The other is Argard’s…
