You do not need a splitter. Connect the LOC to whichever speaker outs you want. At 20k (or more) the LOC presents near zero load to the amp. It will not affect anything. Also the bass being used to drive the subs is so low it is not even stereo. So you could if you wanted connect all the subs to just one channel.
You are right to be thinking about impedance however at 20k ohms or even higher it simply is not a factor.
You are right to be thinking about impedance however at 20k ohms or even higher it simply is not a factor.