Sagging Suspension

[Rat5]

Hello there fam,

I have a question on what I can do to prevent my suspension from sagging. I own a 1989 Mercedes 300E w124. I'm a car audio enthusiast and love to always have a system in my car. And I'm planning a bigger and louder system but my suspension is just bottoming out. Thats with no passengers in the rear seat! I've been talking to people here and there and some say the shocks, some say the springs but I think its neither. I think its time to reinforce the springs.
I've heard and seen some people put like rubber pieces inside the springs. This helps with the weight problem and keeps the car from bottoming out. You still get a smooth ride its just that the rubber fillings prevent you from sagging. Any one know of the name of these? I was looking at the Air Lift 1000 but I think its only for trucks. Any suggestions?

This is what I had. But I'm going bigger!