"I have recently added 2 more 21 inches subs to the existing 2 12 inches subs to my main speakers. I find that the system sound more engaging with just the 21 inches subs. Maybe the subs are out of phase with each other. I do not know how to integrate them."
khiak,
I think you're saying you have four total subs (two 21" subs and two 12" subs) and like the sound with just the two 21" subs and no 12" subs in your system. Is this correct?
Tim