Learning from past experience is a sign of wisdom. And whatever you want to say about Megadeth leader Dave Mustaine, you can’t deny his intelligence. Although he had the vision to launch Gigantour years before the touring circuit became glutted with rock fests, that first trek in 2005 struggled to fill venues despite carrying such proven names as Dream Theater and Opeth on the bill. Gigantour returned in 2006 as a dual-stage event, then trimmed down to one stage and five acts for its 2008 North American run. That third tour was the proverbial charm, setting the stage for Gigantour 2012: It’s playing in a mix of amphitheaters and theaters, and being booked in cold-weather months to avoid getting lost in the usual onslaught of summer concerts.