As press briefings go, it rocked. On Thursday night, Van Halen took to the stage of the venerable Cafe Wha? on MacDougal Street in Greenwich Village, capacity 250. “Last time I stood on a stage this low, we had to have the car back by midnight,” said the singer David Lee Roth. The objective was to prove Van Halen is alive and amicable for an arena tour that starts Feb. 18 in Louisville, Ky.,after the Feb. 7 release of a new album, “A Different Kind of Truth.” It’s to be Van Halen’s first full album with Mr. Roth as lead singer since the band fired him in 1985, although they have done previous reunion tours.