line level crossover for subwoofer

Velodyne used to a make a model SMS-1 which would measure the room resonances via microphone and adjust the bass response accordingly. All is adjustable with the crossover itself to merit individual taste or components. 

 Another alternative is a passive set of devices such as those made by Harrison Labs. In this case, you are on your own choosing frequency, but not slope. Though these products don't seem to be so high end, they contribute little to the twisting of frequencies as do the more 'modern' digital products that I have used. 

     The people that brought us the venerated Dahlquist DQ-10, manufactured a crossover that did absolutely minimal damage, to one's high-pass signal.

   ie: https://sites.google.com/site/mpbarney/home/dahlquist-dq-lp1

      The old (quite literally, like me) Dahlquist guys at Regnar, are still supporting the unit.

    https://dahlquistspeakers.com/dahlquist_speaker_service-20dq-lp1-htm

       I updated, modded, used and loved one, for very close to three decades, with numerous amps and excellent results.

       They've gotten as hard to find as hen's teeth!

      Were you to search for and purchase a unit; one simple (but critical) mod, is to replace the cheesy, OEM RCAs with something like these, which fit the PCB perfectly: 

     https://www.wbt.de/english/products/a/Detailansicht/Artikel/topline-cinchbuchse-3.html  

             As mentioned: hard to find now days, but: well worth the effort!

This HSU high pass filter worked fine for me when I used a subwoofer without a high pass filter option!

https://www.hsuresearch.com/products/high-pass-filter.html

YAWN ….. So Tedious

”My advice is, don't put legs on a snake. You bought an amp specifically designed with the express intent of casting a spell. So, maximize the spell. Pair it with some proper SET speakers like Tekton Perfect SET, that are both high sensitivity and tuned a bit warm to sound better at single digit watt SET levels. ”

+1 JL CR-1. So many others, Bryston, Marchand, First-watt. Really depends on how much $$ you want to spend. Behringer, Ashley, TDM, DBX and many other DSP's I've never tried.

Your idea is decent, my Raven Blackhawk has a sub out option that starts rolling off at 80. Only 3dB/octave but saves the amp a lot as this is where the power is. It does make the amp sound a little better. 150Hz is however way too high. Also Raven implements theirs differently, passively, as otherwise the loss in fidelity would be greater than the gain from the crossover.

Therein lies your problem. You want to take an amp, the magic of which stems from an ultra-simple and pure signal path, and run that signal through a maze of circuits and interconnects. This is gonna be an improvement? All in the hopes of gaining a dB or two more volume? This is gonna be worth it?

I don't think so. I think you will spend a lot of money only to wind up scratching your head wondering why so many guys talk so much about more always being better?

My advice is, don't put legs on a snake. You bought an amp specifically designed with the express intent of casting a spell. So, maximize the spell. Pair it with some proper SET speakers like Tekton Perfect SET, that are both high sensitivity and tuned a bit warm to sound better at single digit watt SET levels. 

JL Audio CR-1

ozzy