I think you will probably find every sub added brings improvement. My initial plan when building my DBA was to sell my Talon Roc to offset the cost of the four DBA. So when the subs went in the Talon went out of the room. The four were amazing! But then just being curious I decided to try with 5. The Roc went back in. Wasn't expecting too much. The literature after all says going from 4 to 5 should only be a very small improvement. What I heard though, or more accurately what I felt, was not small at all! Most amazing of all was the definite feeling that the bass I was getting with 5 was so much lower than anything I ever felt was coming out of the Talon at any point ever.
I think what happens is any one sub on its own, it does go very low but because of room resonances does so at a much reduced level. Then we get a double-whammy because of the way human hearing works, Fletcher-Munson equal loudness counters rise a lot at low frequencies. In other words really low bass has to be quite loud in order to be heard at all. We simply aren't that sensitive to it. But then the instant it crosses that threshold where we do hear it, wow! Impressive!
So more subs increases the lowest frequencies to where we can really hear them.
I think what happens is any one sub on its own, it does go very low but because of room resonances does so at a much reduced level. Then we get a double-whammy because of the way human hearing works, Fletcher-Munson equal loudness counters rise a lot at low frequencies. In other words really low bass has to be quite loud in order to be heard at all. We simply aren't that sensitive to it. But then the instant it crosses that threshold where we do hear it, wow! Impressive!
So more subs increases the lowest frequencies to where we can really hear them.