Do subwoofers really help?

Easy question here all...if you mismatched your amp and speakers (meaning your amp has trouble with the load of the speaker), do Subwoofers really fix that problem? Is it smarter just to sell your speakers and get ones that better match the amp?
I would hesitate to suggest that a subwoofer can cure a 'mismatch' between amp and speakers. It's really only when your amp and speakers are as perfectly matched to each other as possible, that subwoofers can add that extra dimension of low-end foundation whilst seamlessly blending with your existing speakers.
The priority in my experience is to find the 'best' possible speakers for your listening room regardless of the amplifier/s. This is the 'key' to overall eventual satisfaction and enjoyment. If you are lucky enough to accomplish this feat, there will be an amplifier (or amplifiers) that will suit those speakers better than most others and there is fun to be had in finding those very components.
Only when those goals are achieved would I recommend attempting to add subwoofers.
No. Bandaid at best. Change either the speaker or amp. I use subs but run them full range so they have no effect on the amp or speaker.
The only place where "amplifier to speaker matching" would really be an issue in the bass would be with something like a single-ended triode with no global feedback and a resulting high output impedance, an OTL amplifier, or Bob Carver's "current source" outputs with a series resistance.

That has the side effect of increasing output where the speaker's impedance is high, like around the woofer and/or port resonances.

If you high-passed the main speakers+amplifier you'd get the port and perhaps woofer+box resonance out of the amplifier's pass-band which would reduce the problem.

You'd still have problems with reduced output were the impedance was low, like on .5 way speakers below the baffle step. Where that wasn't in the main speaker stop-band you'd still have problems.

Personally, I think that if you're going to use tone controls you should be honest about it and knowingly apply them before the power-amp in the shape tat best suits your room, placement, speaker defficiencies, and tastes.

That's more likely to be successful and cost less than trying multiple amplifier+speaker combinations hoping you'll be happy.
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

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.
Perhaps you should contact Paul Speltz regarding the autoformers before you make any big steps. I have not tried these, but I believe the purpose of them is related to your issues. Based on my assumption that the impedance curve of your speakers combined with their sensitivity and amps' dampening is creating the problem. I don't own or use the autoformers or zeroformers, but I think this is what they are supposed to address.
No, it's smarter to get a better amp. Why would anyone try to match speakers to an amp?

I find speakers I like...and then buy the amp they need (unless my current amp works fine with my new speakers).

Of course, if you have a good pair of speakers, and an amp that can handle their load with ease....adding a subwoofer (or two) is going to make things easier on all components involved....less work everywhere, and more options on placement (the best sound for mid/highs, is not always the best placement for bass reproduction)

I'm not crazy because when I replace my speakers with less sensitive ones (Focals for example) 2 things happen...Bass improves, and I don't have to turn the volume up as high to get good sound. I am an amateur and I did not pay attention to the specs on the speaker before purchasing. The Imepedance curve on the speaker is too steep. I am not buying a $4000 amp just to drive them.

How in the world do you expect anyone to help you when you supply nothing but general information? Do you think anyone here knows what speakers you are even talking about?

And why in the world would you need a $4,000 amp to drive your speakers?

Come on Bob....don't expect everyone to guess your system.

So Focals have more bass in your system. But how do you know the unnamed speakers of your original post are capable of more bass than you're currently getting? Have you experimented with room placement?

As far as impedance matching goes, there are plenty of high current, good-sounding amps well under $4K that can handle a sub 4-ohm load. A Parasound Halo A 23 can be had new for less than $1K. Even their modest $549 Model 2125 has an FTC 2-ohm rating, as does their twice as powerful $1349 Model 2250.
Thanks everyone I found a solution.
I'm sorry.
Am I missing something here? Did BobRock read the many answers to his question?
Replacing speakers with less sensitive ones resulted in more bass?
And just what WAS the solution? I'd like to know, especially if it was something silly like just a setup issue. If he fixed it by throwing more money at it, that's a horse of a different color.
I actually took everybody's advice. The problem was the impedance curve on the speakers was too steep for the amp, but I liked the amp better. So, I sold the speakers and Got ones that better match the amp. The peak output on my amp is 4R. The impedance on my new speakers never drops below 4R so there are no issues. I can tell this was better than adding a sub because now I do not have to turn the dial on the amp as high to play them loud. Sorry for not being more specific I am sure I am not the only person to have done this.

I spent no extra $$ to do this I got good $$ for the speakers I had.