car audio question, quiet speakers?

[Wes]

i have a 2001 oldsmobile aurora, the sound system is all stock, when i turn it all the way up it gets loud then quieter as i turn it. i tryed replacing the 2 front speakers with a couple 6" xplodes with the most wattage i could find. i threw em in and it is the same. do i need a new head unit or whatever and if i get a sub and amp will it have the same problem? please answer with answers and not with... "take it in to a car audio store" because im trying to see if theres anything i can do