If the car had a problem with one deck and still ahs the problem with the new deck....its not the deck. There's a super slim chance you got two faulty decks but I think not. And there's no external amp. Stock wiring in a '91 Si? Thats an old car that been through how many owners? How many decks? Wired by whom? I think its a wiring issue for sure. You checked each individual speaker and they all make that crappy sound so it couldn't be wiring to the speakers. So all that is left is the constant power, key power, and ground. Run each one to their source. I bet its the ground is jacked somewhere between your connection off the deck and the body of the car. It may just be loose where it attaches to the car and the high revs send reverberations through the chassis and cause this?