High Pass Filter for Mains

Many of the active subs have an output for the mains that offer a HP filter. I can’t speak to the quality of them, but they definitely exist, and some might be good. Lots of people do it and are satisfied.  

Another active option that’s really affordable is to use an automotive active crossover for the high pass. The Pyle 2-way is under $40. Add a 12V wall wart, and it’s a go. I’m using one for low pass only, and it’s been great in that capacity, but can’t speak for the quality of the high pass, but I’d imagine it’s on par with the HP options offered in most subs.

Same here, used their 3-way active crossover for years. Just recently sold it. I would have no problem buying from them again.

I went in a different direction, but researched these

https://www.bhphotovideo.com/c/product/752261-REG/dbx_234XS_234xs_Crossover_XLR.html

I don't know anything about them but ASR measured one and it measured better then its price.  

A single cap, and plugging the ports can be very effective.   Hsu speakers also makes custom 2nd order line level

In my speakers, the tuned port is not just low end. It's a tube that terminates an inch from the back of the woofer. Plugging it muffles everything. 

The Merchand filter is 24 db while HSU is 12. It is essentially a single cap for each channel. I was hoping for an off the shelf alternative to waiting a month. I'm impatient.