No I didn't say that at all.HDi, you said to put the LPF off by putting it on the highest frequency. Then you said to adjust it until the bass level drops. But if the LPF is on the highest, when I adjust it it will make the bass level increase...
I said that you need to leave the LPF as open as possible before adjusting the signal level to the sub. (IE THE VOLUME, OR GAIN CONTROL)
Having adjusted the sub level so that the bass seems twice as apparent you then lower the LPF frequency cut-off until the bass level starts to appear marginally lower. By doing this you will find the natural extent of the LF response of your other speakers.
Once this is identified then you readjust the SUB level (ie gain, be it positive or negative) to get what is an even distribution of bass levels between the car's own speakers and the sub.
Can you find some specs for the active sub/power amp combination.
What is the rolloff slope of the low pass filter. This should be quoted in dB/octave.
TN69, you, too, are completely wrong.