My local halfords have a range of speakers on display that you can test out, these being one set. They blow away similar speakers for sound quality. And according to the Halfords site, they're ?44.99+postage online or ~?90 in store, so these are a bit of a bargain as they're brand new and unused.
As for fitting into a ZS saloon, they
should drop right in, although the cover/grille is attached to the OEM speakers rather than the parcel shelf itself.
To upgrade the speakers, you'll need 6 bolts (forget the size) to attach them to the parcel shelf, which will look like this...
and then some sticky velcro pads to stick the grille back down to the parcel shelf fabric
Thats how Ive held mine in, and havent had a problem with them.
Of course, you can forget about the grilles altogether, cos it looks awesome with them
But I reattached the grilles - being in Manchester, I didnt want to see my new ICE go walkies.