When I first tried to set up my sub (REL T9), the result was a hot timing mess, so I let it gather dust. Main speakers are PearlAcoustics Sibelis SG. A year later I gave it another go, and got it to work where the sub is not audible directly, but I notice the difference turning it on on/off. I have no clue what I did differently. I am not interested in window-rattling bass, am actually rather bass averse, yet a bit of low frequency condiment makes the overall musical experience more tasty.
I would venture to guess that the allure of a sub is BASS!!! and that leads to bad quality sound. After all, when spending money you want something for it. However, restraint is key to success. Less is more.