[Klipsch Gain problems Identical Subs]

The audio store where I bought the receiver told me that if I have 2 subs running at the same time that I need to change the phase to reverse on 1 of the subs.  True or False?

[Klipsch Gain problems Identical Subs]

I have a yamaha rx-v1081 receiver with 2 subwoofer outputs. I have two 400 watt identical Klipsch powered Subwoofers model R12SW

I stacked the subs one on top the each and actually liked the sound however I noticed something odd. One sub sounds great at 50 percent gain, but the other sub with the same gain setting is way quieter. I have to turn the gain up to 80 percent to match the bass volume of the other sub. Can anyone explain this to me? Does the yamaha receiver output the same power from each sub output? I tried the subs separated in different locations in the living room and I get the same thing. Both subs sound great, but one is just not pumping out the same volume as the other. The subs are less than a year old and the yamaha receiver is brand new. Please help!!

Thanks in Advance