Odd subwoofer question.

Sub bass is very difficult. It is not just a matter of what you hear. It is more about what you feel. To get the right idea go to a small Jazz club like the Blue Note in NYC. Try to sit dead center of the band. Reproducing this level of sound quality at home is possible but it requires lots of subwoofer, in my case eight 12" drivers, lots of power, each pair gets 2000 watts and digital bass management (crossovers and room control). You have to remove as much bass as you can from the main speakers which will clean them up and increase their headroom by a substantial margin. I cross over at 100 Hz. 

The subwoofers themselves are another world of problems. Keeping the enclosure from resonating is extremely difficult as bass is very powerful. Put your hand on your subwoofer while playing. Feel the vibration? That is resonance and that is distortion. Ideally you would not feel anything other than a cold surface. Building such a subwoofer is very expensive and not viable commercially with only the Magico brave enough to give it a spin with their Q series. Since most of us are not up for spending $40K on a single subwoofer the DIYer is left to his own devices to make it work.  

while @mijostyn 's points are salient to any subwoofer discussion, in my view and experience they are not practical obstacles to having subs (at least rel subs in my personal experience, but i know there are other excellent subs out there) work in a totally satisfactory manner for very high end systems for very discriminating listeners

i would also add that sub bass infomation is also very much about helping the system develop a very palpable sense of space and natural propogation of transient information approximating a real life listening or performance venue

@jjss49 , Absolutely true (the last paragraph)

I have been living with less than ideal subwoofers since....1978. The benefits have always been worth the problems but accuracy in the lower octaves frequently suffered. I have been using ESLs almost exclusively since then with a 6 year diversion into Apogee Divas. Getting the low bass out of these speakers improves their performance by an order of magnitude. I have been chasing a subwoofer system that matches their accuracy since 1978. At this point in time the only commercial subwoofers I would consider are the Magico Q series and they have two major limitations, size and price. I can do better in DIY terms in both size and price. The last set I constructed are about 80% of the way there. The next set should be state of the art. 

The problem with the vast majority of commercial subwoofers is that they are built to a price and take short cuts to make implementing them easier and less expensive. A good example of this is the use of simple low pass filters instead of  full two way crossovers. There are compromises that they have to use in enclosure construction. Put you hand on your subwoofer while playing. That vibration you feel is distortion. There should not be any, even at the loudest volumes. Most main speakers benefit by having the low bass taken away from them to one degree or another depending on how high up into the midrange their subwoofer goes. The higher the greater is the benefit in terms of lowering distortion and increasing power handling. No subwoofer should be crossed over below 80 Hz. If  a subwoofer can not run up to 80 Hz without a noticeable degradation in sound quality it is a poor design. The best drivers are perfectly capable of running cleaning up to 150 Hz. The problems usually arise with the enclosure, crossover and amplifier. I prefer passive subwoofers because IMHE large class AB and A amplifiers make more dynamic bass than the class D amplifiers used in practically all subwoofers. I find it interesting that many audiophiles are willing to go to great lengths isolating their electronics from vibration but are willing to accept an amplifier in a vibration pressure cooker. The problem with high powered class A and AB amps is that they get hot which is highly problematic inside a subwoofer enclosure. Passive subwoofers allow you to use any amp and a multitude of great crossovers to achieve the best results. You can take any subwoofer and turn it into a passive one by disconnecting the internal electronics which is a viable option for someone who does not have the capability to build his own.

… The problem with the vast majority of commercial subwoofers is that they are built to a price and take short cuts to make implementing them easier and less expensive.

Even F1 cars are built to a price point.
And spaceships.

… No subwoofer should be crossed over below 80 Hz. If  a subwoofer can not run up to 80 Hz without a noticeable degradation in sound quality it is a poor design. The best drivers are perfectly capable of running cleaning up to 150 Hz. The problems usually arise with the enclosure, crossover and amplifier.

You forgot port noise. 😁

… on an LFE then there may be good reasons to go from <20 to 80 Hz. And distortion harmonics can quickly get into the localisation freqs at 3rd or 4th harmonic.

Most people have a high threshold for distortion at sub frequencies.
(I am not sure 80Hz is a commandment, 50-120 Hz is a typical “range”, and 60-100Hz likely where the bell curve is thickest..

I prefer passive subwoofers because IMHE large class AB and A amplifiers make more dynamic bass than the class D amplifiers used in practically all subwoofers. I find it interesting that many audiophiles are willing to go to great lengths isolating their electronics from vibration but are willing to accept an amplifier in a vibration pressure cooker…

If there is any place for the efficiency of Class-D… it is in a subwoofer.
I am not sure there is any factual evidence that the Class-A is better for dynamic range than Class-D in kW sized subwoofer applications?

in my case eight 12" drivers, lots of power, each pair gets 2000 watts and digital bass management (crossovers and room control)

That is a lot of bass, and likely over the OP’s budget.
If one can tell if the subwoofer is on, then it is almost too much bass.

(IMO) It should be a bit hard to tell if it is on, but obvious when it is shut off. That hits a nice level relative to the rest of the spectrum.

Assuming that it is mostly used for reinforcing the bass, and not replacing the whole spectrum.