If you mean by rattling.... you can take care of that with sound deadening material...
If you mean wiring, that is mostly in the eye of the beholder and how you want it to look, if you want it to look clean, get some zip ties and make it look clean!
He only has 2 x SVC 4 Ohm subs currently... so 600x1 @ 4 Ohms is out of the question.... It would be 1200x1 @ 2ohms for the current setup.... If he did upgrade to DVC 4 Ohms subs then yes, he could do 600x1 @ 4 Ohms OR 1200x1 @ 1 Ohm....
I thought I read that he hasn't purchased the subs yet so I was just throwing some ideas around.