Ten-gallon hats aren’t typical in Manhattan, but there were practically enough to outfit Dallas on Tuesday night, when about 2,000 fans packed into the South Street Seaport for a free concert by country superstar Tim McGraw. McGraw, who’s married to Faith Hill (who wasn’t anywhere to be seen), kicked off the release of his new album “Set This Circus Down” by playing six songs — some new, some old — underneath a circus-style tent. “We were in Europe for the past three-and-a-half weeks, so it’s good to come back to the U.S. to an audience like this,” McGraw, 34, told the throng. With a backup singer, a fiddler and four guitarists, McGraw sang love ballads laced with lyrics of barbecues, mini-skirts and beer. He had plenty more backup from the audience, which sang along with his old songs and cheered the new ones. As a token of his appreciation, McGraw wiped down a guitarist’s baldhead and tossed the sweat rag to the crowd. That’s devotion.