This was a match made in Downtown theater heaven: an overwhelmingly energetic and intense company in need of some really meaty material (The Amoralists) and the sometimes brutally funny and intelligent — especially with a committed cast — writer-director Adam Rapp. The result, the New York premiere of Rapp’s first full-length play, about a poor mother and her youngest son in the Deep South awaiting the elder bro’s return, doesn’t disappoint.