Yeah, Definitive's stated frequency response numbers are more of a "usable" response range, rather than -3 dB points. My old BP10's have a stated lower end of 20 Hz., but measurements I've seen reflect a -3 dB point of more like 27 Hz. (still pretty low for a floorstander with 6.5" mid-bass drivers).
You are probably not going too far astray if you just use 80 Hz. for the receiver's crossover setting, especially with a nice sub. Even if the fronts can go somewhat lower, your surrounds may have less bass extension, and most receivers don't let you set a different crossover for different speaker positions. (Or maybe also try 60 Hz. to see if you like that better.)