A Y connector shorts two signals together usually so that one signal can feed two amplifiers. Maybe you are wrongly shorting together left and right channels to feed a single subwoofer amplifier? What do you have that feeds what?
Why do you need such a simple RC filter that it very gradually cuts low frequencies below 300Hz?
Usually a stereo amplifier has a 3rd order highpass filter and the subwoofer amplifier has a 3rd order lowpass filter with the cutoff frequency at 80Hz to 150Hz.