Hi Brown Dog,
This is very subjective and also depends on body type, sleeping style, and individual sensitivities so each person can have a very different idea of what “soft” feels like to them. There are also different “types” of softness and firmness that different people are more or less sensitive to (see post #15 here). There are also no consistent firmness ratings in the industry between different manufacturers so in the end soft is whatever feels soft for any particular person regardless of how it feels for anyone else 
When you can’t test a mattress in person then the best approach for your initial configuration or “starting point” is a more detailed conversation with SleepEZ who can help “talk you through” your options and will know more about “matching” the options they have to different people based on “averages” than anyone else (see mattress firmness/comfort levels in post #2 here).
Phoenix