Stiffer rear springs?

[Spinnetti]

Got an ADV 160. We ride 2-up and I'm a big guy. The front fork is fine, but the rear is harsh over bumps, in part because the progressive part bottoms out very quickly. I'd like a spring that is both stiffer and more progressive for better 2-up riding. Anything out there? I don't really want to replace the entire shocks as the bike is brand new, though the rear is under damped a bit too.