Do subwoofers really help?

That all assumes you're not blowing rail fuses, sending the amplifier into thermal shutdown, or getting it to oscillate because it's unstable.

Where it's oscillating you want a better amplifier; and otherwise you may be better served with more efficient speakers.

The underlying question that you pose is "Will filtering LF from the main signal fix the problem?"

There is no way to answer that. The effectiveness of such an approach depends on both the nature of the "mismatch" and the frequency at which you actively filter. Filtering at 75hz may be ineffective while filtering at 150hz may prove entirely effective - or it may not.

The actual impact of the subwoofers is an entirely different question. However, that question, too, will depend (to some extent) on the x-over frequency you choose - so the issues are related. However, I would address them one at a time.

If you want an answer - buy an NHT x-2 on a money back in-home trial basis. Filter the LF to your mains without a sub. If this relieves the distress of your main speakers, you can explore subwoofers. If it doesn't, you can return the X-2 and swap your system out.

Good Luck

Marty

I think a sub is a band aid in the scenario outlined. The right approach (happy path) is to fix the speaker/amp mating first and go from there.

Yes, especially dual units.

What is the nature of the 'mis-match'?
Impedance of speaker too low for amp?
Phase angle problem......amp can't produce enough power?
How can you tell there is a problem? distortion? Smoke/flames?/ Neighbors complain? Dog leaves room when you start the Stereo?
How did you end up with this combination? What's going to prevent you from doing it again?

Fixing this problem by buying MORE stuff is probably not the answer. You can do as others suggest, add complexity and drive yourself nuts with fine tuning / messing about. OR..... get a proper amp if you love the speakers or proper speakers if you love the amp.