The sub output is a fixed 200Hz low-pass filter. Which likely isn't rolling off the bass of the speakers when connected or otherwise, you will need to go into the settings of the amp and adjust the 100Hz "bass EQ" levels down to -6dB as needed, depending on how loud you need the LGKs to play.
You may be able to get away with no EQ most of the time, but the more you adjust it down the louder they can play so long as you adjust the crossover and volume of the sub to compensate.
For a somewhat budget option, the Rythmik L12 as a solid option for a sealed sub.