Help! What notch/dash is what frequency on my home audio subwoofer gain knob? 

I use my ear to set the filter. But you can guess fairly accurately: if the speakers are, say, 3" bookshelf, in the range of (predominant) 200 Hz to 16000 Hz, set the sub to about 150Hz. If the speakers are large, set the sub to a lower pass, say, 80Hz or lower. The sub fills the void of the woofers.

Mine dont have any notch at all it starts at 50 go to 170 that's it there are no markings.