You can absolutely use one sub and have an improvement in sound, however...
"I feel I am missing some oomph and I find I am turning up the volume a lot." Maybe you should be looking at other parts of your system also. What kind of speakers, how sensitive, how much power are you giving them? If your speakers aren't up to the task of filling your room with the sound level and dynamics you want, a subwoofer(s) won't fix that.
A single subwoofer if properly implemented will add more body to the music, but it's not going to add much volume, at least it shouldn't or you will be overpowering the rest of the music.
A single subwoofer may sound good in one spot in your room, and not so great in others because of peaks and nulls (too much or too little bass). You will probably need to do some time doing the "subwoofer crawl" to make sure it sounds best at your listening position.
Adding another sub, ideally having 4 subs, helps cancel out those peaks and nulls.
You don't have to have super expensive subs if you go the route of multiple subs. Adding additional subs means that you can play each individual sub at lower output, giving them more headroom. My music system started out being more focused on home theater.
I have 4 subs, two have twin 15" drivers and 800 watt amps and two have 13.5" drivers with 1000 watt amps. Two have analog volume controls which are at about 9 o'clock, the other two have digital volume controls and are at about -15 on the volume. In other words, they are barely turned up, yet when the music calls for it, there's a visceral instantaneous reaction from the subs, while most of the time you might not even notice them.
My point being that you don't have to spend a lot if you get multiples and you don't have to get super expensive subs either. I'm not naming names, but Rel for example. Wait, I did name names. I've heard Rel subs and they work great, but so do many other subwoofers. There's nothing special or unique about Rel subs that I have heard. I'm sure fanboys will say they weren't set up "right" or I'm deaf, or whatever, but that's my story and I'm sticking to it.
If I were to make suggestions, I would say for sure go with sealed subs and if there's a brand that gives a good combination of price/performance/sound quality, to me, it's Rythmik. I have and enjoy subs from SVS and Power Sound Audio also, but I like the servo technology in Rythmik subs. If there's anything "special" about Rel subs, it's that they are sold by dealers, so it is easier to hear one, but that also means you're paying the dealer's markup compared to buying direct from the manufacturer like the other brands I mentioned.
