Im going to tell you what I noticed when going from playing only 2 subs due to not having the proper amp to push 4 . When playing 2 Clarion SW1051 subs rated for 300 rms@ 200 watts in a box made for 4 subs the bass was lound, but often not defined for certain music. I almost didn't buy 2 more because when turned up to the 3/4 volume setting it was almost unbearable to listen to for long periods of time. IE 3 songs.
Now keep in mind, the amp pushing the 2 subs is now pushing 4 infinity reference 6922cf speakers. Before hooking up the 4 subs with the new amp, I hooked up the highs/ mids and played a few tracks and almost decided to say forget the subs because of the clarity of the music. I enjoyed the clarity for 3 hours or so.
On the same day I installed the new amp which I believe is holding it's own pushing the 4 Clarions @ 325 watts a piece, 2 ohms per channel. At first the highs floaded out the bass which made me think i had a dud amp, but after adjusting the volume of the highs to compliment the lows, the system sounds like I have a decent home rack system in my car. THERE IS ABSOLUTELY no engine noise or whine. No hissing or clipping. I also have true bass control, when some songs require more or less.
IMO 2 subs give you a fuller sound and helps fill in the missing depth of the bass you my experience when using one sub. To put it in plain english, 2 would sound better than one if set up properly. And so on and so on and so on as far as adding subs.
WB