Hi Littlemanb,
This can be somewhat of a challenge to find the best balance between the competing needs each sleeping position because the requirements of each position are different … especially stomach and side sleeping. Stomach sleeping tends to need a firmer surface to prevent hammocking of the pelvis into the mattress and side sleeping tends to need a thicker softer comfort layer to allow the shoulders or hips to sink in in more for pressure relief and to fill in the curves in the body profile for side sleeping and to a lesser degree for back sleeping. In these cases it’s usually best to choose the thinnest firmest comfort layer (or topper) that “just barely” provides the pressure relief you need on your side so that the risk of alignment issues is less on your stomach.
Other things that can be helpful is a zoned support system with a firmer zone under the pelvis to help prevent the pelvis from sinking in too far relative to the other parts of the body (allowing the use of softer materials on top with less alignment risk because you will sink in more evenly), using a pillow under the pelvis when you sleep on your stomach, or a body pillow which can help you sleep more on your side with the “feel” of sleeping on your stomach.
The key in most cases though is to make sure that a topper is not too thick so that your hips are closer to the firmer support layers. 2" would be “safer” than 3" in most cases for people who spend significant time on their stomach.
Choosing the most suitable topper without testing the specific mattress/topper combination can be almost as difficult as choosing a mattress but post #2 here and the posts it links to have some guidelines for choosing a topper which should be helpful.
This may have been a matter of the thickness of the topper rather than the type of material. A thinner memory foam topper may have worked well because it will allow the support layer to “stop” your pelvis before it sinks in quite as far. A topper would mostly negate the benefit of the cotton or wool mattress cover though.
There are three different types of foam (latex foam, memory foam, polyfoam) so I’m not sure which type of foam they are suggesting (I’m guessing memory foam because you mentioned “agan”?) but in general the choice between the different types of foam would be based on your personal preferences because they are very different from each other in terms of how they feel and respond. Regardless of which type of topper you choose though … I would make sure it was soft enough to relieve pressure on your side (extra firm would only be more of the same as what you already have) but thin enough to keep your pelvis from being too far away from the support layers.
As you mentioned … this is a lot to pay for a latex topper and I would consider talking to some of the sources listed in post #21 here which ship across Canada and where a latex topper may be available for less (assuming you prefer latex over memory foam). Costco also has a Talalay latex topper here which is 3" (may be a little on the thick side) but is also a little firmer (medium firmness which may compensate a little for the thickness) which may be worth trying. If nothing else this would give you a reference point for a latex topper with little risk because of Costco’s return policy.
Phoenix