I use three Rel subs (two T5's and one T7) plus one SVS PB2000 along with a Musical Fidelity M6si integrated as well.
So you know, 4 subs are better than two and putting the subs right next to your speakers is risky at best. If you are experiencing low frequency issues without the subs, it may be due to your speakers being in a less than ideal position for bass reproduction. Placing the subs near to that position may do nothing to help because the subs will be in a bad position as well. Do a sub crawl and find where the subs work best.