My guess is that the 300W Kickers are not RMS rated, otherwise your 45W/channel amp would have all kinds of problems driving them.
I replaced the ones in my ML with Jaycar 85W 6.5" and was quite happy with the result, running on a 50W/channel headunit amp. Alpine make some very good speakers, but you have to listen to them first because the sound varies, a lot, from model to model. The top end Clarion are pretty good too and then there are a whole range of top shelf speakers. In the Pajero I have Sony splits in the front and Panasonic in the rear. Not 100% happy but replacing them again isn't on the priority list at the moment. Again, listen to them first - sometimes they just don't suit the music you like to listen to. The Sonys sounded great in the shop but the 50W/channel headunit amp just doesn't have the grunt to drive them.
Have a look at
this thread and read my comments on the factory speakers.