Had a quick look and you should be able to wire up the whole system including a sub using the jbl gto4000 amp that is in the link you posted.
I like JBL kit never had any problems with it good solid sound and JBL - 'Just !Removed! lLud!' 'nuff said
The 6x9 speakers you have, from what I have found, are 2 ohms so depending what the rating of the sub is you run the risk of overloading the amp if you choose to bridge in conjunction with the four speakers as it specifies a minimum of 2 ohms per speaker line so if you then bridge the rear connections using the subwoofer it could take it below 2ohms
So if it was me I would have that amp doing the four speakers - at 2 ohms that will give you a constant 75w of sound power, with the 937's being able to handle 100w constant that should give a nice sound. Whatever you go for upfront you can go 2 or 4 ohm, 4 ohm will give you slightly less constant power at 50w but you may prefer that if they are closer to you
Let us know what the other amp is and I can do some more digging