Summing and Lowpass active filter for LM3886, this OP-AMP correct?

Not really.

U2 & U3 - why are you summing the output back in?
U3 & U4 - may lead to instability as you are asking for less than unity gain as frequency rises, also the response will be droopy and to make a 2nd-order active filter there are proper ways that only require one op-amp.

Take a look at the sub-woofer controller project on my website, it is exactly what you need.