I’ve read a lot of these posts but not all of them, and I’m not nearly as knowledgeable as everyone here. My experience: you’re better off with two less capable subs than one more capable subs. I’ve got two 10-inch subs. I started with one ten inch and added another 10 inch and I felt a lot more confident in things.
If you are using it for music you are going to use it for the bottom octave or two of music. It will take a big load off of your receiver or amp if you use a crossover. The sub will most likely handle everything from 80 hz down better than your mains if you get semi decent subs. The addition of subs means a big increase in fidelity of recording reproduction from 80 hz or 60 hz on down below 40 hz.
For music 40 hz is a pretty critical area. If you’re good at 40 hz with no rolloff there it’s usually a big upgrade in sound. I personally would recommend an amp or receiver that handles the corossover characteristics for you. I cross over at 80 hz for the mains, you can take it down to 70 or 60 hz if you are paranoid about sound localization at low frequencies.
I would not run the mains at full frequency range in conjunction with subs, I’d cross over at some point one way or another.
It’s nice to have some digital eq for the subwoofers in specific and some room eq used in conjunction with you ears and common sense. I personally would concentrate on positioning of the subs first and DSP and eq second.
I prefer closed subs rather than ported. As I understand it if your main intent is movies then ported subs are better according to a lot of people. Handling movie soundtracks with aplomb requires a much more expensive subwoofer than one used mainly for music. I focused on music.
You should keep in mind that it gets pretty difficult to perceive pitch in 1/2 step gradations once you get below 40 hertz. So below 40 hz you are really trying to fill in and get realistic visceral sound but it doesn’t have to be super accurate, IMHO, if those super low frequencies are even in the music or the recording.
I tend to focus on what I am trying to get musically rather than focus on the technical audio side. A typical double bass in classical or jazz or rock (either electric or acoustic as the case me be) hits a low note of 41 hertz (an open E on the lowest string) or so. If the recording reproduces this having the subs give you that note cleanly is a big deal, IMHO. A lot of modern music has some synthesized bass below 40 hz, there are some electric double basses that have an extra string and go a little below 40 hertz, lots of organs do, pianos do but it’s largely obscured by harmonics. But a clean 40 to 80 hertz is pretty critical musically.
A lot of recordings and speakers deliberately emphasize harmonics above the lower fundamental frequency tones on purpose so it will still sound nice on normal gear. (Your brain infers the lower note from the harmonics.) But having your system actually hit the lower fundamental frequencies sounds really nice. Adding subs is often one of the most efficient ways to improve the actual fidelity of an otherwise well-functioning system, IMHO.
Don’t over-estimate what you need to spend on a sub or subs if it’s mainly for music at any kind of reasonable listening level. It’s too easy to get caught up in differences in capability you will never use just because measurements of those capabilities are out there.
End of brain dump.