What is wrong with a sub?

1+ wolf_garcia. mkgus as long as you keep it symmetrical you will be ok.
If you use two subs up front they should be identical. If you were to add two more subs they could be from a different manufacturer. The DSP is great but you have to know what you are doing and to do this you need to be able to measure the situation with a calibrated mic. You can not change the nodes in the room. You can only correct the frequency response at your listening position. If the listening position is in a nul point
you want to move it back and forth a little in the room to where the bass is if anything too loud. In a nul point it will cost you a lot of power to correct. This is easy to do. Get a test record or download with frequency tones. Play a 40-60 Hz tone and walk back in forth in the room right between and 90 degrees from the speaker axis. You will hear the bass come and go as you walk in just a few feet! Mark the spot you like and move the listening position to it. Now correcting the response will save you power and you won't need a 2000 watt amp.

I within last year discussed subwoofers for music with a Wilson and REL dealer, who said that even the Wilson ALEXX, a phenomeonal $110k loudspeaker rated to 20Hz, benefits from well-done subwoofers.  The benefit is subtle, but it's there, according to his crew as determined in A/B testing in their listening room.

There are many good subs on the market. Just get good controls for the subs and consider how many you really need and placement. I have speakers that go below 30 but I tried one good sub and then, over the years I moved slowly to four. There are reasons for this but you will learn them on the journey. IMO there is a better sound with four subs than most other setups. And I am not talking wall shaking; I am talking immersion and realistic sound.

I bought a smaller REL i7.  I was thinking maybe I should go bigger. 
Assuming the same total $pend...
Seems several think having several smaller might be better than one bigger.  I expect bigger can go lower Hz.  If this was for a home theater, maybe I'd get the earth quake / bombs better with a single bigger sub, but for music, which is what I want, that little i7 REL is low enough - I think, then I don't have the bigger ones.

Thoughts on more smaller vs one bigger?

The simplistic statement by, once again, our eric squires, says it all. Purchasing the " correct " ones, and the " set up ", is the key. Subs ( Wawa, makes excellent Hoagies, and this is what they are referred to ) are needed in almost every situation, ime, but getting them to " match and blend " with the main speakers, and the room, is the ticket. I tried running my Lascalas, with a crossover, but based on my listening, I prefer them to run full range, and allow them to taper, naturally, on their own, and bring my built pair of subs in, at that point, where it is the most seamless, to me. I do not hear the subs as a separate entity. Took a lot of time, and work. Test tones with spl measurements can be a wonderful tool, as well as selecting a recording or two, that you feel, is truly wonderful, at that " transitional " point, between the mains, and subs. There are subs, that only excel with home theater, and subs, that only excel with music. Of course, there are subs, that excel at both. Anyone who simply plops down a sub or two, might find it great, but believe me, adjusting, and readjusting, tweaking and some more tweaking, can truly get you phenomenal results. Enjoy ! MrD.