I own a Klipsch RSW-12 Subwoofer and a Def Tech Powerfield 15 Plus. When I connect the second subwoofer(Def Tech Powerfield 15 Plus) to the
line out(Klipsch RSW-12) it seems like both subs put out less
bass. Why is that? Is there a way to fix this? I even tried a y-cable from the subwoofer output on the receiver. The same thing happens. I think the preamp level/voltage from the receiver isn't strong enough for both subs.
Panasonic TC-P50ST60 plasma HD television, Onkyo TX-SR805 and PIONEER ELITE VSX-47TX receivers, Klipsch RB-75(2 pair) and RB-61 bookshelf speakers, Klipsch RSW-10, RSW-12(2), Velodyne HGS-12, HGS-18 subwoofers, PS3 80G, PIONEER DV-525 dvd player, Klipsch RS-42 surrounds, Klipsch RC-52 center channel
Even if your receiver has no output level adjustment for the subwoofer, (if that's true put a new receiver on your Holiday shopping list ), there should be adequate output voltage to drive two subs -- if connected in parallel (as you are doing with the Y connector) each sub input will still see the full output voltage, perhaps less a small amount because of the lower total input impedance of the two subs in parallel.
A more likely scenario is that you have acoustic cancellation between the two subs. If either one of the two subs has a continuously varable phase control, try twirling it while bass is playing -- if just 0 or 180 degree switches, try both positions.
Your strongest bass, and your best blending of non-identical subs, and your easiest way to minimize acoustic cancellation, is to co-locate the two subs -- stack them or run them side by side.