Subwoofer for music, how low?

Sounds like you got it figured out. Servo or sealed, powered or ported, get whatever you like just get 4 and you will be fine. Then if you also are able to budget for Townshend Pods under those subs, and Podiums under your speakers, this will greatly reduce room resonance even more while giving a large improvement in clarity, detail, and 3D imaging. The combination of DBA with Pods gives a feeling of envelopment, of being in the recording venue not merely having it in front of you. 

A big reason for this is speakers and subs put a lot of energy directly into the room. Being coupled to the floor vibrations excite the floor and walls. The whole room physically resonates. Isolating speakers greatly reduces this. You have to hear it to believe it. Like adding a pile of tube traps but without the wasted space and with improvement in detail you just can't get any other way.

If you need to save money you can get a good share of this with spring isolation like Nobsound springs. These are not nearly as good as Pods but a lot better than you would think for how cheap hey are. Of course they can be used in combination. Speakers on Podiums, subs on Nobsound is a good compromise if you need to save money.

With DBA you will be amazed how easy it is. Set them different distances from corners (asymmetrical), set crossover below 80, set levels, and you can tweak it from there but already it will be very, very good! 

Based on a rectangular room, with multiple seating this is what Todd Welti’s research showed (I assume his research was more geared towards home theatre application). Google Todd Welti if you’re not familiar with him.

"It can be seen that there is less difference between “good” and “bad” seats, and that there is less low frequency support for some configurations compared to when the grid is centered and unweighted. Two and four subwoofers at the wall midpoints are still the best configurations overall. Four subwoofers in the corners does not seem as advantageous as when the grid was centered. There is still little or no advantage to using a large number of subwoofers. LF factor still goes down for higher numbers of subwoofers."

"CONCLUSIONS • How many subwoofers are enough?

Four subwoofers are enough to get the best results of any configuration the best results of any configuration tried. Two subwoofers is very nearly tried. Two subwoofers is very nearly as good and has very good low as good and has very good low frequency support as well."

In other words, you may be wasting your money having more than 2 subs, and certainly wasting money with more than 4. The best outcome is between 2-4 subs, but you need measurements to determine which is best. Is it worth spending money to fix a 2-3dB dip/peak in your room by adding a 3rd or 4th sub?

I think you’re better off starting with 2 subs (especially if your listening environment is single sitting for music), spend $100 on a good mic and some cables. Take some measurement using REW (free software) to see if more subs are required. You could save yourself a couple of thousand dollars.

Dear @golfnutz  : 1+ .

Two subs are enough to listen MUSIC at one seat position, the key is " one seat position ". Normally to listen MUSIC we all need only one seat position that's normally is the best position not only for the bass range but for the mid and high frequency ranges. We have to take care what is happening at that seat position because is the one we use to listen MUSIC in a stereo way.

R.

Who cares what happens out that precise seat position ?

First than all mi and high frequency ranges are the main issues to put in position the speakers in a way that  between the seat position exist a mate/married.

Any one of you can make a simple test: move 1" the angle/position of one of your stereo speakers and you will know what I'm talking about.

Listen MUSIC in a stereo system is something personal/individual that we can enjoy it at maximum only at one seat position because MUSIC is not only bass but a wide frequency range.

R.