actually my friend, if you cared to view the picture of the amp, you will see that it allows LPF on both 1+2 and 3+4
inside the red circles are the filter settings, inside the yellow settings you can see where it says "LPF" on both sets of channels.
set both of these to LPF
then, set the lowpass filter dial to the 80mark on all of the filter dials.
set all of the low boost dials all the way to 0. you dont want these on, coz it will cause distortion
leave all the level dials where they are.
so you need to bridge a sub to each pair. so one sub to 1+2 and the other the 3+4
you will need to split the RCA cables though, from the single pair of subout, to 2 pair. one for each channel.
then use the 2 chan to run the front speakers