Subwoofers masking other frequencies.

Bought a third sub,a svs sb2000 to add to my existing 2 subs hoping for stronger and more detailed bass response. My mains are magnapan 1.7i driven by parasound A21 +. After messing around with  different ways to intergrate the subs I realized some parts of the music was missing or reduced in volume or one channel was stronger .well I turned off one sub at a time and the sound was full. Is this common and what's the remedy for this phenomenon. 
Thanks