The s / subs are the door speakers (in the 9x6 series).
Yes the amp has a built in crossover and only sends lows to the "subs" (using the term loosely) not full range. Not sure of the cut off point but it's there.
Also, on the input side, the subs are driven off a combination of the signal from the front and rear channels so you will need to hook both up from the h/u to get correct function of the "subs". Just thought I'd pass that along too...
Good luck
