Sounds like it's not set up properly. Possible that:
1) the crossover point is set too high so that it is doubling up frequencies already output at good volumes from your main speakers
2) the volume is still set too loud
3) the placement is bad
I recently added two subs and found that an SPL meter helped a fair bit to verify what I was hearing. I don't have true flat response but I have been able to get it close enough that my system definitely sounds richer without the bass playing any louder than the rest of the music.
Bottom line - keep experimenting - this takes days, not hours...
1) the crossover point is set too high so that it is doubling up frequencies already output at good volumes from your main speakers
2) the volume is still set too loud
3) the placement is bad
I recently added two subs and found that an SPL meter helped a fair bit to verify what I was hearing. I don't have true flat response but I have been able to get it close enough that my system definitely sounds richer without the bass playing any louder than the rest of the music.
Bottom line - keep experimenting - this takes days, not hours...