mains and subwoofer phase issues

If it ain't broke don't fix it. You are just reversing the absolute phase by connecting hot to ground [or vice versa] , it can't do any damage. Reversed often sounds better anyway; if I liked the sound I would leave it alone. You can change the subs if you feel comfortable doing so but you don't have to. I run my subs out of phase with the main speakers, which way sounds better depends on the system and room.

I agree with Stan -- if you are happy with the sound leave it as is. Absolute phase reversal on the main speakers will be inaudible on most recordings, and on the few recordings on which it may make a difference, the difference will be minor.

However, what COULD do damage would be reversing the hot and ground input leads within the sub. You didn't fully describe the setup, but if the sub has its own amplifier, and it is connected to the Cambridge's speaker output terminals, and both the sub and the Cambridge have 3-prong power cords, then depending on the design of the sub a reversal of its hot and ground inputs could result in shorting the Cambridge's hot output to ground, which would definitely not be healthy. And likewise if you are connecting line-level outputs of the Cambridge to the sub.

Regards,
-- Al