NICK KREWEN Hamilton Spectator May 25, 1996   For almost 26 years, Ian Anderson has sung loudly and carried a big flute as frontman for British progressive rock group Jethro Tull. A few years ago, however, the 47-year-old made a shocking discovery: after playing the wind instrument on 28 Jethro Tull albums, he’d been using the wrong technique. “I’d say 80 percent of my fingering was incorrect,” says the…