As one of the more significant speaker design advances we've seen in recent years, Gear4's new DUO (£150/approx. $290) is a two-piece audio system that becomes bigger or smaller depending on your needs. The metal and plastic front piece is a portable, rechargeable battery-powered portable audio system with a fold-out dock in front and a fold-out stand in back, while the considerably larger plastic subwoofer sits on a desk waiting for you to attach…