Hi glennk,
Just in case you haven’t read it yet the first place I would start your research is the mattress shopping tutorial here which has all the basic information, steps, and guidelines that can help you make the best possible choice.
There is more about the different ways that one mattress can approximate or “match” another one in post #9 here. I certainly wouldn’t consider a mattress that used similar quality materials to any of the mattresses you are considering because most major brand mattresses tend to use lower quality materials in their comfort layers than I would be comfortable with considering … see this article) but quality and durability aside … how a mattress “feels” is the most subjective part of comparing mattresses and a mattress that “feels” the same for one person may feel very different to someone else.
Most manufacturers don’t make mattresses that are designed to be equivalent to mattresses made by another manufacturer so I don’t know of another mattress that is specifically designed to feel similar to any of the mattresses you listed so the only way to know whether any other mattress would feel the same to you (regardless of whether it would feel the same for someone else) would be based on your own personal testing either in “real time” or with very little elapsed time in between testing both of them (our memory of subjective comparisons such as “feel” don’t last very long). When you spend time testing mattresses where you aren’t able to find out the quality of the materials inside it then in most cases the time you spend testing it is mostly wasted (see this article) unless you plan to buy the specific mattress you are testing (which I would definitely avoid if you can’t find out the quality of the materials inside it).
I would also avoid using another mattress as a “target” (which may not even be the best match for you in the first place) and rate every mattress you consider against a common set of criteria based on the parts of your personal value equation that are most important to you.
There is also more about the most important parts of the “value” of a mattress purchase in post #13 here that can help you make more meaningful comparisons between mattresses.
Some Therapedic mattresses may be worth considering on a “mattress by mattress” basis and the odds of being able to find out what is inside one of their mattresses is higher than many other larger brands (see here) but this will also depend on the knowledge and transparency of the retailer that is selling them. The brand name of a mattress is mostly meaningless though because outside of PPP (Posture and alignment, Pressure relief, and Personal preferences) a mattress is only as good as its construction and the quality of the materials inside it … regardless of the name of the manufacturer of the mattress on the label (see post #5 here and post #12 here).
You can see my thoughts about liquidation outlets that sell used mattresses without a warranty in post #8 here and post #2 here and post #2 here. I would tend to avoid them.
The better options or possibilities I’m aware of in the Dallas/Fort Worth area (subject to the value guidelines I linked) are listed in post #4 here.
Phoenix