I got 1991 LS400 all stock pioneer stereo. except for the front side panels are boston acoustics

I want to keep my head unit, but i want 2 add a few subs into my car.

I have no idea how to approach this and what to buy brand wise.. i was thinking to look for an amp to replace the stock amp and another amp for the subwoofers, if i need one... once again.. i have no clue on how to setup this thing... btw, money is an issue, dont really want to spend an arm an a leg, but i could sell my factory sub, which is only a year old..

any help please, thanks

-Peter