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.
