dual svs sb12 nsd subs - Page 2

This definitely sounds like a phase problem, like ceh said, switch the phase setting on ONE of the subs, not both, and see if it sounds louder. You are probably getting cancellation, which is where the peaks and troughs of the sound waves from the two subs are opposing where they meet at your listening position, and they can cancel each other out, leaving you with less SPL than just running one sub. As for the Klipsch sub, I haven't seen measurements for it, but I would guess it has a peakish frequency response with a lot of mid or upper bass but not so much deep bass. It will be made for big output at more easily perceivable frequencies, which is not what the SB12 is made for.

Well, if the second sub out IS backwards setting the sub plugged into it's phase to 180 should also correct the issue.

Is it possible that SVS sent you an SB12nsd with the speaker leads reversed? I had this issue on two occasions with my SB12nsds where one was out of phase with the other. As per SVS the best solution is to remove the amp and switch the + and - leads as switching the phase from 0 to 180 doesn't completely fix the issue.

I would think not, as both subs plugged into 1 sub out with a y adapter seems to have fixed the issue....