Let's face some facts. The KEF LS50 don't do bass well. On their own they lack bass extention and begin to distort. You are compensating for their inability to produce bass and dynamics by pushing the sub into upper bass territory @ 160hz.
IF you had speakers that could properly handle bass down to say 50hz then your subwoofers could be adjusted properly to give a dynamic low end and operate as they were designed.
Bottom line... if you want great bass and dynamics these aren't the right speakers. The subs are fine. No need to buy another one.
These speakers are great for near field, low level listening for classical music. But if your looking for some punchy Jazz tunes or electronic chill music the little drivers in these things can't handle it.