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! 

No, there is no "music" at 20Hz- well not much anyway. However, there definitely is musical content that low, and it makes a huge difference. Mostly it is the lowest ambient resonance of the recording venue. This is the super low frequency throb you feel at a concert hall, but it is there on a lot of other recordings as well.

Very hard if not impossible to accurately reproduce with only one or two subs due to room modes and cancellation, it is easy to do with four in a distributed bass array. The result is bass like you never heard before. Not just bass you notice with music, either. Way better than that. When this ultra-low frequency ambience is accurately recreated it makes the system and room disappear leaving you with a greatly improved sense of envelopment. The music is not spread across the room in front of you any more. You are now in the recording venue.

The beauty of this method is it no longer depends on finding a super powerful sub that goes down low. In fact you can use any four subs, they do not need to match, can be literally anything. Perfectly suited to your situation as you can simply buy whatever 4 subs are readily available in your area. Rely on the fact that using four will outweigh any considerations of what they are.

I have by the way done all the different permutations to know the above is all true. Tried a whole bunch of subs, one at a time. Killed myself moving them around. Tried in vain to get good bass. Cannot be done. Forget about it. When you hear people say they got great bass from one or two, ask if they heard four or five. So how do they know?

I have one Talon Roc powered sub, two sealed 10" and two ported 10" passives run with the same Dayton SA1000 amp Duke uses in his Swarm. Duke helped me building mine. I am returning the favor passing along what I learned. This gives me 5 subs, and I have tried them all kinds of different ways. Killed myself so you don't have to. Just get four, any four, you will see.