When you think about it, the fact that Radiohead is one of the biggest bands on the face of the planet sorta defies logic. How is it that the architects of such bracingly complex music are so, well, popular — when the vast majority of the population who get pleasure from pop in its least complicated forms doesn’t seem to have shrunken one bit? The same question could be directed at the Punch Brothers, a band that has taken bluegrass instrumentation through a labyrinth of both classical and indie rock-spiked compositions under the leadership of mandolin-playing Chris Thile. They ask a lot of their audience, and it results in quite a heady ride. But even a quintet of acoustic virtuosos may want to streamline things once in a while, which is exactly what the Brothers have done on their arresting third album, Who’s Feeling Young Now?, which was recorded locally at Blackbird Studios with Jacquire King. They aim for penetrating, tuneful efficiency with youthful romantic angst, and they hit that target head-on.