Hi ann,
Unfortunately, like many of the members here that were in a similar situation, you are in a difficult position where you need to exchange one lower quality/value mattress for another because higher quality/value mattresses aren’t available to you and you are locked into an exchange at the store you purchased from.
Post #2 here and the posts it links to has some suggestions that can help you make the best possible choice out of the options you have available and they will boil down to either choosing a mattress where you can confirm the quality of the materials in the upper layers (which is the weak link of most mattresses) or choose a mattress which has the least possible amount of lower quality or unidentified materials (which will often be firmer) and then add a higher quality topper that won’t break down as quickly as a comfort layer that used lower quality materials. This also has the advantage that you can just replace the topper when it softens or breaks down more quickly instead of replacing the entire mattress.
Post #1 here has most of the basic information, steps, and guidelines you will find most useful and would also be worth reading (even though some of the steps won’t apply to you because you are exchanging a mattress not buying a new one).
I would also not pay much attention to any online reviews which are usually written shortly after a purchase and are written by consumers that usually have no idea of the quality of the mattress they purchased and because what works “perfectly” for one person or even a large group of people may be completely unsuitable to sleep on for the next regardless of the quality of the mattress. You can read more about reviews in post #13 here.
Again I wouldn’t pay a great deal of attention to reviews. You can see more information about the iComfort lineup in post #11 here and in the case of the Savant there is only about an inch of lower quality/density material in the upper layers (the “comfort foam” which is low quality polyfoam) which is within the guidelines I would normally consider OK although it’s not great value. The top layer is good quality memory foam and the gel memory foam under the 1" polyfoam layer is medium density/quality gel memory foam. It has comfort layers that are very soft though and it could be a risky choice for a back or stomach sleeper … especially when the memory foam softens a little.
Hope this helps and of course if you have more questions along the way feel free to post them.
Phoenix