An OBS 1500 would have squatted just as bad. I had a piano in the back of my obs before the drop and it dropped it 4-5" at least. I swapped in my hotchkis springs hauled the same piano, and it squatted 1/2".
Stock 1500 springs are just made for comfort. IMO.
Do you like the ride of your Hotchkis spring's, I have never used any of their stuff yet?