Limiting a full range driver

OP:  Sounds like a good idea, but, this is key, be ready to measure your results.  Once a speaker/subwoofer gets into a room throw the specs out the window and pretend you know nothing about the performance of the speaker, because without measurements you really don't.  The room changes everything so while I think your approach to using digital miniDSPs is sound I also think you'll need to carefully gauge the setting and for that you'll need a mic and software.

Well, the usual recommendation to avoid subwoofer localization and enhance the ability to put it elsewhere is to keep the low pass filter somewhere below 120 Hz.  I haven't heard any other methods.

Hi OP,

Doppler distortion is a real thing, but usually the room interferes more.  It's worth understanding if your bass is smooth enough before going much further.

The quick answer is you can put a high pass filter in front of your amp, which is just a single capacitor at the right value for the amp impedance. 

I think 250 Hz is a little too high to cross a main speaker at.  Around 100 Hz is usually the upper bound, 60 Hz the lowest I'd normally recommend, but the idea that adding a subwoofer AND utilizing a high pass filter on your mains is sound, especially if you have room modes which you want to use an EQ to tame.  This way you put the EQ in line with the sub only.

Also, check out the AM Acoustics room mode simulator which will help you understand how bad your room modes are as well as give you ideas for the best placement.