@rauliruegas is right in many ways with a few caveats.
In a normal point source system two subwoofers is fine and more does not add much. Line source systems require a driver every 4-5 feet along a wall to form a line source subwoofer to match the main speakers.
I would not bother with subwoofer drivers smaller than 12". 10" drivers are fine if you use two of them per enclosure. Most big point source systems require two 15" subwoofers. The smaller the driver the greater the distortion levels at a given volume.
Blending the subwoofers into the main speakers using second through 4th order slopes is appropriate for all subwoofers except a small number of truly excellent units. Subwoofer enclosures are extremely hard to make. Most of them are musical instruments and have a particular sound. If you use a steep slope with these subwoofers the cross is very noticeable. Turn the volume up and place your hand on the sub enclosure. If you feel any vibration you have a musical instrument. Subwoofers that have no sound of their own (do not vibrate at all) can be run higher up using very steep slopes 8-10th order (digital only) as there is little sonic difference between the subs and the main speakers at the crossover point. Here the important factor is keeping the sub out of the midrange where you will hear it. Subs are integrated correctly when you can't tell they are there. Using a low pass filter only on subwoofers is a sad proposition. Easily 1/2 the benefit of using a sub is cleaning up the midrange of the main speakers. You have to use a high pass filter on the main speakers to do this. It is not done because of the added expense of electronics and cables.
I believe all subwoofer systems should run down to 18 Hz. Music does not go that low but venue clues do. Large venues breath at very low frequencies. With some live recordings you can feel the venue. Subwoofers should be felt more than heard. This is the difference between a live and reproduced performance. Proper subwoofers give you the feeling and excitement of a live performance.
In my mind there is no difference in the requirements of a theater vs music system. A great music system will sound just fine in theater mode and is quite capable of rattling your teeth.
These are some important concepts to keep in mind when buying subs. Balanced force design. Some of the vibration you feel is the enclosure shaking in opposing response to cone movement. Placing a driver at opposite ends of the enclosure cancels out these forces lowering the musical instrument factor. Smaller enclosures are better because they are easier to control lowering the musical instrument factor, but they will require more power and possibly EQ. A box is the worst shape for a subwoofer. The best shape would be a sphere, but this is impossible to make. The next best shape is a cylinder. Cylinders have inherently stiff walls and are less prone to resonance than flat walls. Cylinders are relatively easy to make and I am surprised that nobody is making cylindrical subwoofers. Open baffle subwoofers are IMHO a silly concept and a florid waste of power. Why some people think they are great is more an indication of how poor many enclosed subwoofers are. There are many excellent subwoofer drivers out there. Dayton makes a Reference sub driver for any situation and I do not see how you can do any better. DIY by all means. Think big pipe with a driver in each end. You can get aluminum pipe in almost any dimension, cap the ends, drivers, binding posts and cleverness.
Subwoofer drivers take a beating and even the best ones fail on occasion. It is not a bad idea to keep a spare driver handy. The drivers evolve and it is unlikely you will be able to get the same driver after 10 years. Without a spare you will have to replace all of them.