Sounds like destructive interference. You'll need to adjust delay/phase/position to ensure the sound from all the subs and your main speakers arrive at your seating position at the same time. Room EQ Wizard can help by measuring each speaker and subwoofer individually, relative to each other (see https://www.minidsp.com/applications/auto-eq-with-rew/measuring-time-delay for an example).
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
Thanks