When the Feelies tore into their opening song at the Music Hall of Williamsburg Saturday, it was without even a “hello,” despite the seventeen-year hiatus that they only recently ended. The silver foxes in black Converse who made up the audience greeted their heroes with silence, too — a reverential one. Dressed like Friday-casual office drones, the band was surprisingly agile for a group that’s been broken up for longer than they were active, and never fond of playing live in the first place. Who knows what vitriol might’ve lurked behind front man Glenn Mercer’s black shades, but he seemed to enjoy blazing through his band’s economic, almost ascetic back catalogue — more than once he jumped, with both feet coming off the ground! He didn’t even seem to mind that there were people watching and bopping their heads. (There were even reports of some scattered jostling during a couple of numbers.) Though the only time any band member dropped to his knees was to fastidiously tune his instrument between songs, the old material sounded more raw and rough than on the recordings. Mercer even allowed himself to stretch out on guitar, blessing his followers with some distorted, bluesy riffage. The Feelies’ caffeine buzz hasn’t waned with age and time off; if anything, they’re more pumped than they ever were.