Hi jbrennan,
You can read more about the previous versions (up to version 4) of Pranasleep mattresses in post #3 here and in post #2 here and in post #3 here and in posts #1 and #2 here and a forum search on Prana (you can just click this) will also bring up more information and feedback about them as well.
The good news is they changed the design in version 5 and the top layers of polyfoam are now higher quality/density and are no longer what I would consider to be a weak link in the mattress in terms of durability but they are still in a significantly higher budget range than other similar mattresses and the top layers are still polyfoam so you would still be feeling and sleeping on polyfoam as much as latex. There is more about the new version 5 in post #19 here.
You can see the specs of the Pranasleep Super Vinyasa Luxury Plush here (click on overview) and as you can see there are no lower quality materials or weak links in the mattress since they started using higher quality polyfoam in the top layers but I would still make some very careful value comparisons with other similar latex mattresses that use the same type and blend of latex (such as the Wazee) and are in lower budget ranges.
Outside of PPP (Posture and alignment, Pressure relief, and Personal preferences), a mattress is only as good as it’s design and the quality and durability of the materials inside it regardless of the name of the manufacturer or the price of the mattress. I would always keep in mind that the price of a mattress often doesn’t reflect the quality or durability of the materials inside it. I don’t know the specifics of the materials in the Dunlopillo Lux Firm so I can’t make any meaningful comments about it but if you can find out the information listed in this article about the materials in the mattress and post it on the forum (the retailer should be able to provide this to you) I’d be happy to make some comments about the quality and durability of the materials and the mattress as a whole.
Phoenix