Why ?

The car was uses to haul heavy amps and guitar cases for several years and even though the new struts improved the ride, she seems to be hitting the bump stops on rather smooth roads with no load and just the driver.

I'm thinking just the rear springs... what do you think?

Yes, if constantly overloaded the springs can become compressed. The struts dampen the spring oscillation, the springs handle the cars weight.

I had new springs installed and the difference is amazing, I have new springs for the front and I'll install them soon.