Instead of throwing money at speakers that are unlickely to give much of an improvement, consider 'Dynamatting' specific areas of the car, which will improve the overall response of the speakers. I'd sugget doors and door cards plus the shelf where the subwoofer resides (saloon) or wheel well and sub enclosure itself (Avant), to start with.
Use a butyl based product, like Silent Coat mat, as it's more effective and lighter than bitumen based products, like Dynamat (original).
Or if you want 'real' bass, a better subwoofer (with dedicated amp) will be the best route. You're still better off adding some deadening, if you go that route anyway.
Paul