It depends on what you're doing music and venue-wise. I have the Jeff Berlin, a NY 151, and two NY 121s. I made a top for the combo head, such that it can slide in and out of the combo (likely voiding the warranty). I've tried all the combinations. The two 15s make for a huge, deep sound. I did the 121 with the Berlin for a while, and it was a great sound. Initially, I had some trouble because I was turning the 121 on its side to fit better with the 15. That caused some kind of buzz, but turning it right side up fixed it. I'm running two 121s now and am loving it for the sound and portability.