My experience adding subwoofers to 2 channel

avanti1960,

     You may want to try the 'room crawl method'.  Place your sub at your listening position, play some music with good and repetitive bass and walk around the perimeter of your room counter-clockwise starting at the front right corner.  Walk slowly listening for an exact spot where the bass sounds best to you (accurate, detailed and natural).  When you find this spot in your room, just move the sub to that position and verify the bass sounds good from your listening seat. You can use your RTA at this point for fine tuning if you'd like.
      If you prefer good bass response throughout your entire room, and not just at your listening seat, the only solution I'm aware of is a 4-sub DBA system.

Tim

Tim,

Thanks.  I'm going to start a thread on this topic.  

Cool.

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.

"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