When you say you do not like the x-over point, is it higher than you would like? I am going to tell you a few things: 1. My experience with active and passive filters. 2. A basic explanation of what you are asking. 3. Try it out so you can see( hear) it for yourself.
I first used an inductor when I was 15 and could not afford a better amp or active x-over...... I NEVER used one again. They are just not versatile. I say that kindly. The main problem is the amount of slope a single inductor can give you. the crossover slope for an inductor is 6 dB/octave. What that means is that a lot of unwanted frequencies get through. HOWEVER, if you use a bandpass enclosure, 4/6 order for example, now an inductor works well.
Are you using the pre-amp (rca) outputs from your deck to your sub amp? Well if so, how would you then connect the inductor to the rca.The inductor has no rca connections. An inductor goes in series with the subwoofer on the output side of the amp. I have heard of ( but never used) a passive line level x-over, and that goes before the amp, but it's not a single inductor, it's a few components eg.. capacitors, resistors, inductors, etc...
My suggestion to you is this: Connect your sub setup how you have intended. Set the amps x-over at full range and use the inductor (after the amp). Test it out, tweak your gain, face your sub in various directions till you find which way gives you the most cabin gain, basically just see how it hits. After that, take the inductor out of the equation, and set your lp filter and run the same test. This time tweak your lp filter setting as well as all the other adjustment I mentioned above until you find the best sound. I'm confident that you will like the way the lp filter on the amp performs versus the inductor.
Honestly, it would take a 300 pg book to get a decent grasp on filters, active, passive, digital, etc. These things are seriously complex. They involve things like phase shift, amplitude change, dealing with fluctuating impedances and blah blah blah. Seriously, it's gets very confusing very fast.