Hi peterr,
Post #2 here has some suggestions that may be helpful when you are caught “between a rock and a hard place” and need to exchange a mattress at a mainstream store.
If you are going in the direction of a complete mattress (rather than a mattress topper combination) then careful and objective testing for PPP following the guidelines in the tutorial post will be very important and I would err on the side of firmness because all mattresses will soften over time and it’s also easier to fine tune a mattress that is a little too firm by adding a topper than it is to “fix” a mattress that is already too soft because this would involve removing or replacing some of the layers instead of adding to them.
The iComfort Prodigy may be a reasonable option if it is a good “match” for you in terms of PPP because it uses mostly good or reasonable quality materials in the upper layers (5 lb memory foam, slow recovery latex, and 4 lb gel memory foam) with the exception of 1" of what they call comfort foam which is low quality/density polyfoam. 1" is within the guidelines that I would usually consider to be OK for low quality materials and wont have a significant impact on durability unless the mattress is already on the edge of being too soft for you in which case even relatively small amounts of foam softening can put you outside of the comfort/support range that works for you.
You can see some of the specs of the Comforpedic Balanced Days here (click specifications) but I wouldn’t consider it unless you were able to find out the type and quality/density of all the layers that are listed. Without being able to confirm the quality of all the layers it would be a risky exchange IMO because you would have no way to assess any weak links in the mattress.
Phoenix