I'm building a cab for 4 Eminence Beta 12's. What tuning frequency and cubic feet should I do for a *smooth* response down to 35hz or so? It's for a 4 string player, but I don't want 40hz for the E string to limit him. He may play in drop D occasionaly. Not looking for flat response. Just a slow roll from around 90 to 35hz. Any suggestions?