Hi carpediem,
I’m not quite clear whether you’re adding a topper to the mattress you already have which appeared to need a little more softness or if you’re starting all over again.
Post #2 here has some links to some generic guidelines that can help provide some insights about design choices but these are only generic and not specific to any individual. There are too many variables and unknowns to make comfort choices based on “theory at a distance” and the best way to choose is always your own personal testing and experience on similar mattresses or alternatively more detailed phone conversations with a manufacturer who know more about their mattress designs and options than anyone else and who can give you some guidance based on the their knowledge and experience, the information you provide them, and the “averages” of their customers that are similar to you in terms of body type, sleeping style, and preferences. Of course by definition not all people fit inside “average” ranges so your recourse if you make the wrong choice is also an important part of your personal value equation.
As you can see from the generic guidelines … when you sleep in multiple positions … the most effective choice is generally “just enough” in terms of the comfort layer softness to provide pressure relief in your most pressure prone position (usually your side) so that the risk of alignment issues is less in your “flatter” sleeping positions such as back and especially stomach which is the riskiest sleeping position of all.
If I’m understanding you correctly … your previous mattress was 6" of medium Dunlop (you didn’t mention the type of latex) and the wool topper (not sure if you still have and plan to use this?).
Your current mattress has 6" of medium Talalay which will be a little softer than the Dunlop and then it also has another 3" of soft Talalay on top of this. This is quite a jump from one side of the softness/firmness scale to another. I would be a little cautious making this kind of a jump although of course I wasn’t part of the conversation where you discussed this in much more detail so I don’t know if there are other factors that may have contributed to such a big difference in your choice.
In general … just enough softness and thickness to relieve pressure on your side is less risky for back and stomach sleeping. I would talk to them a little more to make sure they are aware of the construction of your last mattress and that you were looking for something just a “little” softer. What you have would be closer to a lot softer and may be on the risky side … especially if you spend a lot of time on your back and especially your stomach.
I certainly agree with you that Arizona Premium is among the best value available which of course is why they were invited to become a member of this site 
Phoenix