Do subwoofers really help?

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

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.