Have you run the preload collars all the way up? If so, then get a different (softer) spring, such as a 100 or 90.
Do some research on setting the rear sag on the quad. Ohlins did testing on this last year and they gave me the numbers as to how much sag the quad should have for MX. You may try calling them and speaking with the quad guy. He is hard to get ahold of.
How can zps make a softer ride? The small spring will be compressed already so the main spring which supports the quad will still have to be compressed on the initial compression. Wouldnt a dual rate spring be the way to go with as shorter mainspring and maybe a zps on top of that for lowering.