Hi Diogo, I'm sorry to hear that you're experiencing this problem. This is an SP-16, correct?
A quick way to really test this for sure is to use only a single 1/4" cable and test it on each of SP-16s audio inputs to listen for a discrepancy. Even better is to go into the sampling area and look at the volume meter for the incoming signal. Plug into the L input, look at level...then plug into the R input and see if there's a difference. Do this process once with the filter on, then the filter off. Please let us know what happens and we'll go from there.