Hi latexhappy,
Adding a topper to a firmer mattress has several advantages and few disadvantages over a mattress where all the layers are inside a finished cover that can’t be opened (doesn’t have a zipper).
For example … the Pamper (that has 1" of 19 ILD talalay built in) with a 2" 19 ILD talalay latex topper on top would be very similar to a mattress that had a 6" 40 ILD talalay latex support core and a 3" 19 ILD latex topper inside the cover. A topper will be a little bit softer than the equivalent layers all inside the mattress because it can move independently of the rest of the mattress (similar but more so than a pillowtop which has “free” edges and more independent movement). A topper will also be be a little less durable than the equivalent layers inside a mattress because it wouldn’t have a mattress cover which will slightly reduce it’s compression (depending on the cover) and would be subject to more mechanical compression and wear.
The advantages would be similar to having a zip cover which would give you access to the inside layers of a mattress. These are that you can customize the sleeping system (mattress + topper combination) by choosing different thicknesses and softness/firmness levels for part of the comfort layer (in this case the topper). Toppers and zip covers with loose layers inside also gives you the ability to replace just the top layer of the mattress (which will be the first to wear out) without having to replace the entire mattress.
There are also many “pathways” and layering combinations that can provide similar degrees of pressure relief and support in a mattress and with a good choice of an appropriate topper it can be a good way to “design” a mattress for your needs and preferences. Modular designs … whether they are loose layers inside a zip cover or a mattress/topper combination are an increasingly popular “sleeping system” because of the flexibility and choices they provide. The separate 2" and 3" toppers that are made by PLB are part of their modular design (they can be added to any of their mattresses) but any topper will provide similar benefits and the PLB are more expensive than other similar topper options.
Bear in mind as well that for the price of a Pamper alone you could buy an all latex mattress with a zip cover that already has the comfort layer built in. Post #21 here will give you some alternatives here. There may also be local manufacturers near you that sell a mattress similar to the Pamper (or even just a 6" core with a ticking/quilting over it on either one or both sides) for less than the Pamper.
Phoenix