Hsu has custom high pass filters

Interesting and affordable. Where is this wired into the system? 
 

@zlone

Here ya go!!

Congratulations! You have purchased the high pass filter that would help keep the deeper bass out of your main speakers. In order to get optimum performance from your new purchase, please read this easy-to�follow instruction manual for the high pass filter and the manual for the subwoofer before trying out the subwoofer. It can save you a lot of time and possible unjustified disappointment. A/C Connection Connect the power cord from the wall adaptor to the high pass filter and the wall adaptor to an AC outlet. You can leave the high pass filter permanently powered. If you need to unplug it, unplug it after the main amp has been turned off. Connecting to Your System The high pass filter is for use with our subwoofers that does not have a high pass filter. For example, if you have the VTF or STF-3 subwoofers, or the TN1220HO with the 250W amp, this is what you need to keep the deep bass out of your main speakers. It is not suitable for use with the STF-1 or STF-2 both of which has only a single line level input. The high pass filter goes between your preamp and your main amplifier. Connect from the left and right outputs of your preamp to the inputs of the high pass filter and from the high pass filter to your main amplifier. Connect from the second set of preamp outputs to the subwoofer amp. If your preamp has only a single set of preamp outputs, use Y connectors to get two sets of preamp outputs. Get single male and two female Y connectors and plug the male ends into the preamp outputs. That gives two sets of female preamp outputs. Functions of Controls EQ Switch – EQ1 position gives you the first high pass filter frequency. We usually set that for the lower of the two chosen frequencies. EQ2 position is for the higher crossover frequency you have chosen. For example, if you have chosen 60 and 80 Hz high pass filter frequencies, EQ1 will be 60 Hz, EQ2 will be 80 Hz. Stereo/Mono Switch – Leave this in the stereo position. Subwoofer Level Control – When used as a high pass filter, this adjusts the gain of the main speakers. Set the level control to about 1 o’clock position for unity gain.

Thanks @yogiboy, I guess it won’t be a fit for my REL 7i which I connect to my speaker outputs. 

@zlone No it won't. Line level only!

To the point of slopes, I think THX had the best idea when they attempted to use the speakers + active crossovers to achieve Linkwitz-Riley 4th order filters.  The idea is that the sealed satellite speaker, crossed at 80 Hz would naturally produce a 2nd order Butterworth filter.  Add another such filter, at the same point with the same slope and you get a combined LR-4th order.

This is cool because now you can mate it directly with a subwoofer with a 4th order LR filter too and have perfect matching. 

Of course, many reasons why we can't do that today, and we rely on DSP in the subwoofer chain to deal with any discrepancies.