A week may be a long time in politics, but it’s aeons in dance music. Stand still too long and you risk being left behind. London-based duo Lamb developed an idiosyncratic take on drum’n’bass, offsetting its crisp, tachometric drive with sweetly soulful vocals on their 1997 debut. They’ve made slight adjustments over two subsequent albums, but still see no good reason to mess with a successful formula. Consequently, their fourth casts them as confident modern classicists, as adept at skittering, Dave Brubeck-like tones (“Sugar 5”) as a Debussy-inspired instrumental (“Angelica”). Wisely, Lamb let the bandwagons roll by without them.