Thanks for your help very much (I don't know how I didn't find out polarity...)
It's interesting (for me) that now the polarity of the subs are different, and now the sound is very good in the middle. Is it normal? I mean, the polarity is normal at one sub, and changed at the other.
It is unclear what you mean by that statement. If the output of each sub
is in polarity with regards to the other one, they will sum together and be louder than a single sub. If the output of each sub
is out of polarity with regards to the other one, they will still sum together, but be less bass than a single sub.
You need to check your wiring from mixer output to subwoofer input to determine that it is all wired correctly to maintain the same polarity throughout the signal chain. If you have had to change the polarity to make the subs add together positively rather than negatively there is something wired wrong.