You have options.
Cheep way would be to get spring helpers from your local parts store. Though that would not be the correct fix.
You could replace your factory shocks with after market air shocks and set the hardness to your needs with the air PSI
You could also buy heaver duty rear springs
If you wanted to spend some money and really help your car out would be to find a suspension off of a wrecked 1996 or newer Bonneville that has the factory installed FE2 suspension
How many miles on the car again?
You probably have a big speaker box in your trunk to right?
If you do think about spending even more money on higher end sound equipment.
High quality sound doesn't always have to be heavy!