I'd be astonished if the displacement of one 12" sub could damage the 6x4"s... Just think of the air pressure when you slam the boot shut. If you were running them off a headunit (especially with a "loudness" control switched on) or loud without high pass, that's the usual cause of damage - clipping killing the voice coils.
I can't give you any brand/product recommendations... May be worth getting on one of the ICE forums with those sort of questions.
Obtuse as the statement may sound, your best 1st stop may be sound deadening... The rocco has a lot of road noise, and no sound deadening in the back 1/2 of the car... You'll need a lot less sub bass then. It's amazing how much noise there is below 100Hz -if you drive at 60 and set the sub to the "right" level, then park up, switch the engine off and listen again, you'll see what I mean. If you can get away with needing less bass overall, your choice of small woofers may be increased!