Ah, the never-ending story of system enhancings. I would recommend you to change your speakers for JL audio factory replacements. Never ever let your installer drill some new holes in your car! you'll never be able to take your speakers with you, and it will lower your car's resale price. There are very good factory replacement speakers too from Pioneer and well, many other makers.
As for the Riviera having been discontinued... regrettable. How are the prices for a 98' model? maybe I could make an import, never mind I'm full of debts (If my wife ever reads this it'll be hell back home)