In 8-10k, I would suggest you start by first upgrading the pre. A decent xplod unit will cost you about 5-6k. If you are sticking to the 10k budget, just focus on your pre and speakers. An amp won't fit in that budget. At least not a decent one anyways.
If I remember correctly, cultus has 4" speakers in the dash? If that is the case, You can buy some 6.5" speakers from Pioneer's A range and either custom install them in the front doors or on the rear rack.