a graphic eq has fixed center freq. ie: 20hz, 35hz, etc. a parametric eq has user adjustable center freq. ie: 20-80hz, etc.
most eq's will have a rca input, though i haven't seen any with optical inputs/outputs.
some will have a sub and speaker level control, others just have a sub level control.
this clarion unit has a sub, and main level controls, (sub has a switchable lowpass at 60 or 90hz @ 12db/oct), has a aux rca input on the back, and has front, rear, and sub rca outputs:
this hifonics unit has 2 rca inputs, preamp, switches for highpass(front/rear speakers)/lowpass(sub), 3 band eq(low, mid, high), and sub, main vol, and fade:
this earthquake one is very similar, and has a 3.5mm aux jack in the front, and rca aux in the back(though i think they are tied to gether internally)http://www.sonicelectronix.com/item_...EQ7000PXi.html