Hi genagoodrow,
Impressions are part of the “normal” response of wool toppers or any thick layers of wool over time (unlike foam where impressions are a sign that it is breaking down and has lost its resilience). In some ways you could say that it is somewhat like memory foam except it takes on the general or “average” shape of your body more permanently. It is less resilient than foam and will compress by about 30% or so and then stay in that range so it doesn’t distribute weight as evenly over the whole surface of the body like foam does and tends to provide more localized cushioning that is more pressure point specific. Even when it is compressed it will be less firm than fibers like cotton which are less resilient and become even firmer over time as they compress. This will also depend on the type of wool (courser wool stays more resilient), the density of the wool fill , and on the tufting of the wool. Some of the most costly natural fiber mattresses in the world develop impressions which is part of their “normal” response over time.
When this happens … in most cases the difference is not big enough to cause pressure issues on a mattress that has some softness underneath but in some cases it is (especially if the layers under the wool are firmer) and your idea of having a softer layer to provide some “give” under the wool is a very good one IMO. This would let you keep the “feel” and breathability of the wool without the pressure point issues.
Latex is the most durable of all the foam materials so there would be less of an impression or “soft spot” issue than with other materials. Your experience with your latex is somewhat unusual and certainly not the norm although I also have to say that impression issues with latex have been a little more common over the last few years because the biggest supplier of latex in North America … Latex International … has had some sporadic quality control issues in that time where their latex has impressed or softened to a greater degree and more quickly than what would be considered “normal” (more than about 10% or so would be crossing the line). This has caused some degree of consternation among some mattress manufacturers and even put some manufacturers off of latex itself even though it is less likely to happen with some of the other latex manufacturers that seem to have more consistent quality control. The impression issues you had with your Plushbeds could have also been partly due to the wool in their quilting or it could have been an issue with the quality of your latex. If you put a bare latex layer on the floor then use a string or a straight edge from side to side you can more easily the depth of any impressions in each layer of the latex mattress to see if there is more than a slight impression. Either way … “good latex” doesn’t develop any significant issues after 3 months and only forms slight impressions in the “normal” course of events.
Overall … I think your idea is a good one. While there are too many variables and unknowns to use any “formula” to choose the best thickness and softness for any specific person . The guidelines in post #2 here and the posts it links to can help you use your own experience on your mattress as a good guideline as to where in the range or thickness and softness would be best for you. An “average” choice would be in the range of 2" or so with a softness that was appropriate “on average” for your weight, sleeping positions, and preferences.
Some of the better options in the Annapolis area are listed in post #2 here but personal testing for toppers may not be as accurate or effective for a topper as it would be for a mattress because how it feels and responds will depend on the materials and design of the mattress it is used on as much as the specifics of the topper itself. Unless you test the the topper on the mattress you are sleeping on or something very close, it may feel very different from what you test it on when you put it on your own mattress because they will interact together to produce what you feel. If you can’t test the combination (or something very similar) in person … then buying a topper online would make sense … especially if there was a good return or exchange policy in case you make a mistake in thickness or firmness level.
Overall I think your idea is a good one and "in theory"and based on “averages” should work well.
Phoenix