I bought a Markbass CMD 121H combo in '08, thinking it would be the end-all for me. But, no. Earlier this year I decided I wanted to go with dual 115s instead, with a separate Little Mark III head. To begin the change I first bought a Markbass Traveler 151P cab and used it with the 121H combo as an extension. I knew it was a mismatch, but that was ok for now since I know I will eventually get another 151P cab and separate head (actually, I already have gotten the head now). Anyway, it was interesting to hear what the rig was like using the 12 and 15 together with the LMII head that is in the combo. Oh yes, it sounded great, much better than the 12 alone, I mean, a whopping much better. But, I knew that I was limited by what the 12 can handle, so I was never getting the best use of the 15. And, adding another matching 151P cab will no doubt sound better.
My point is, adding any extension cab will give you a better sound. But, after listening to those in the know on here, I am convinced that when I get another matching 151P cab it will sound even better, and will be the best I can do. I thank those who know the science better than me for their input on the subject. I'd have never known if I hadn't been on this forum.