Are you certain the springs are "bouncy"...or is the ride bouncy?A Bouncy ride is often mistaken for weak springs when actually stiff springs cause that sensation. You need to test the bounce...push down on one corner and see if it continues to bounce or if it raises slowly. Chances are are that the springs are too stiff and the "helper springs" would soften the ride.
Try to increase the spring preload, that will help for start...