Yes, staid old Philadelphia – where just 12 years ago poor Michael Hersch had a major premiere booed; where, as recently as 1986, fully half the audience walked out of Pelléas et Mélisande, for Pete’s sake – has started embracing contemporary classical. There’s Opera Philly’s O17 festival, the new October Revolution fest, a daring series at the Barnes Foundation, plus now-established outfits like The Crossing and FringeArts; next spring there’s even going to be Stockhausen’s 14-hour chamber-music cycle Klang. There are four serious new-music events in just the next week. David Patrick Stearns explains how the change happened and talks to some of the key players involved.