Hi howardsbabygirl,
Thanks for sharing the results of your “Philadelphia” experiences with us.
It seems that you may know more about foam mattress materials … at least from a technical point of view … than the merchants you were visiting 
Magniflex is an Italian manufacturer that has some unusual and in some cases high quality mattresses but of course they also carry a premium price. They use foam from GommaGomma which makes some very high density and quality polyurethane (watergel), memory foam (extrapur) and latex (latexcel) in both slow and fast recovery versions. Magniflex mattresses are also Oeko-Tex certified for VOC’s and harmful chemicals which is unusual for a mattress manufacturer which uses polyurethane. It’s always important to be able to verify the specs of each mattress you are considering from the retailer though to make sure it has no weak links and to make meaningful comparisons and informed choices and its a shame that they couldn’t provide this information.
Croydon is also a little unusual as well but in a different way. As you can see in my comments here … they are not as open (or knowledgeable) about some of the materials in their mattresses. Hopefully, as more people ask them they will decide to become a little more more transparent. Some of their mattresses use fairly thick layers of polyfoam in the quilting and I would want to know the quality of these layers because the top layers of foam (either memory foam or polyfoam) are usually the weak link of a mattress.
There are many manufacturers that are not as technically oriented but in some cases they have been making mattresses for so long that they have the experience to tell a good quality foam just from “feel” and its weight and in many cases they have been using certain foams for long enough that they know from the feedback of their customers over many years which foams are more durable and which aren’t. Foams are changing all the time though so its still important to know the density of the layers even with an experienced manufacturer … and it also allows more meaningful comparisons with other mattresses.
I think my overall view of Croydon is that they are better value than most mainstream mattresses but not in the same value range as many of the smaller or local manufacturers.
Well you certainly have it narrowed down
I would also keep in mind that there are very few manufacturers or retailers that will know the compression modulus of their foams and I would tend to go by density as a main indicator along with the feel of a mattress. If the mattress has 2 lb polyfoam in the base or better, it will generally be a good quality and durable material. HR polyfoam is much more unusual to find in the base layers of a mattress and it is also more expensive than HD foam. Even Tempurpedic uses 2.2 lb HD polyfoam in their support layers and their base layers will last for a very long time. The comfort layers are where foam quality is particularly important because these are the layers that wear and soften the fastest.
I think you are well on your way. The only other guidance I would give you … because memory foam can be so varied … is to use the 4 lb as a minimum guideline but if you come across some 5 lb memory foam that has the performance and feel you want (or gel memory foam for that matter) then I would not exclude these either. In the case of gel memory foam I would want to know the type of gel memory foam they are using (see post #2 here for more information on gel memory foams if you do decide to consider these). You may also want to consider a polyfoam base in the 2 lb density range and higher (so you don’t exclude some good quality/value mattresses looking for true HR polyfoam which you may not be able to find.
It’s also nice to know that if all else fails that there are some good online choices that will meet your guidelines and your local testing will be very valuable if you do decide to go in this direction.
I’m looking forward to more of your feedback.
Phoenix