Human hearing is generally regarded as 20 to 20,000 Hz, though the high end in particular tends to vary according to age and damage to hearing from loud noises (which is often confused with age, as most damage to hearing from loud noises does not show up immediately). However, at the low end, you can feel frequencies below 20 Hz, so there is a reason to want them, too, if one's budget allows for it.
How much material you are missing depends entirely upon what is in the recordings to which you are listening. Many modern soundtracks have bass going below 20 Hz, and pipe organ music can go below 20 Hz.
In my opinion, having very deep bass adds significantly to the experience. I used to have a subwoofer that was flat down to 28 Hz, but I replaced it with subwoofers that are flat to about 15 Hz. The difference is significant, and I am glad I upgraded. However, bass that deep does not come cheap, so you need to consider how much you are willing to spend before you decide you must have very deep bass.
To get an idea of "reasonably priced" subwoofers that go deep, take a look at:
http://www.svsound.com/
Here is their least expensive subwoofer, flat to about 20 Hz:
http://www.svsound.com/products-sub-box-10nsd.cfm
At that price point, you probably cannot buy a better subwoofer.
If you are curious about such things, you can go to a good audio store and listen to various subwoofers. However, SVS is online, not at regular audio stores, so you will be listening to different brands.
As for hooking up more than one subwoofer, you can use a Y as you say, but you need not worry about the impedance of the subwoofers (with powered subwoofers, you are dealing with line level input impedance, not the impedance of the actual driver). However, you will have more difficulty in setting things up properly if they are not identical subwoofers. If you are wanting really deep bass, I recommend that you simply replace your current subwoofer with something better, rather than try to use it with another subwoofer.
