The last 2 channels should be "sub-out" so make sure you connect the RCAs for the sub amp to that. Don't use Y-splitters unless you've run out of channels on your deck, which you haven't yet.
I'd hook up the subs to a 4 ohm final load and use the Sony. For both of those subs, I'd be using a 1kw amp, but see what 500W does for you...could be good enough if your box is right.