Adjusting the controls on the subwoofer amp for best performance is critical; a mis-adjustment can easily make much more of a difference than the variations among competent subwoofer brands. Here is the sequence I generally suggest:
1. Adjust the level of the sub(s) to blend well with the mains;
2. Adjust the lowpass filter frequency of the sub(s) to get a better blend;
3. Adjust the phase to get an even better blend;
4. Go back to step 1 and cycle through these steps a couple more times.
This sequence was suggested to me by an award-winning acoustician, Jeff Hedback. These steps are included in the setup guidelines for the subwoofer system I make.
Setting the level is especially critical because the ear is quite sensitive to differences in SPL at low frequencies. A 3 dB mis-match at 40 Hz is comparable in audibility to a 6 dB mis-match at 1 kHz. And to make matters worse, the relative level of bass energy on the recording can vary significantly from one recording to the next. So it is not unusual to occasionally tweak the level of the sub(s) long after the initial setup session.
Duke