TTA radio improvements

[LSTTYPE]

Hi all. I have had a tta now for about a year and I am going to keep the factory radio and steering wheel controls. It sounds horrible right now. Nowhere near the sound that my Buick put out with concert sound (86 Ttype). What speakers do you guys reccomend for the factory radio that would improve sound quality? It has 6x9s in the rear/sides and 4x6 speakers up front. Thanks in advance