Like dre said 80hz is best and a good place to start with. But some installed midranges can't play that low without nasty rattles (doors without proper deadening) or just dont play loud enough low enough (not enough power, speakers not designed to play that low, incorrectly installed, etc).
Start with 80hz and then try 100 or so. 80 is best if your system can swing it. But some sound better at 100 or higher. Just play around and see what you like the most.