OK, I'm a newbie so go easy. Planning my DIY sub, I find that below about 19 Hz the driver goes into excursion overdrive. Hence as is commonly recommended a HP filter at just above this point is a good idea. Can someone tell me if at a 4 ohm sub impedance and wanting a crossover at 20 Hz, why I cannot just install a 2000uF cap (2200 available) as a passive filter after the amp and before the driver. I know (after reading ) that the driver will vary with impedance at different frequencies but at this low a figure won't it be pretty stable at the rated value? Can I just do this? Any advice would be appreciated. Thanks, Anthony.