Hi Beeg,
The PLB 3" topper is very soft (14 - 15 ILD) and in combination with the 2" of 21 ILD on top of the Pamper (which is also in the soft range) the odds are high that your comfort layers 90the combination of the topper and the 2" layer in the mattress) is too thick and soft to provide you with good alignment … particularly for back sleeping. A thinner and slightly firmer topper would have better odds of working better. The 2" topper would likely be an improvement but it is still very soft and you would “go through” it easily … especially with the heavier parts of your body … so there would still be some risk of alignment issues. Now that you know the “feeling” of comfort layers that are too thick/soft and the more subtle signals that your body sends you … you would also be in a better position to test the 2" topper and Pamper together using the testing guidelines linked in post #1 here to see how much of an improvement it would be but in general I would tend towards slightly firmer choices unless your testing indicates otherwise.
While it’s always a good idea to give any change some time before making adjustments so you can rule out any possible causes for your symptoms that are more temporary or transitory … I think that the odds are quite high that what you are sleeping on is too thick and soft based on your feedback. Sleeping on the Pamper by itself is a good idea IMO and then you can use your experience on the mattress by itself to help you decide on how much more “comfort” you may need using the guidelines and information in post #2 here and the other posts it links to (and if you can return the topper it could also save you some money as well).
Phoenix