Hi goitsme48,
Bedinabox uses 3 lb memory foam in their mattresses … not 8 lbs.
I would be very careful about the other two mattresses you’ve listed because they are likely to contain lower density memory foam (under 4 lbs density) in their construction. It’s important to know the density of all the foam layers … not just the top couple of inches and in the case of both of these manufacturers I would check the density of all the layers with the shipping weight they provide to confirm the information they provide is accurate.
Post #6 here and post #5 here would also be worth reading before you consider any of the mattresses you’ve listed.
A forum search on Dynasty and on Lucid (you can just click both of these) will also bring up more comments about them.
Some of the better online memory foam options I’m aware of are listed in post #12 here.
I would also be aware that with memory foam in particular that there are many different versions of memory foam and each of them can feel quite different from each other even if they are the same density (see post #9 here and post #8 here). How each one feels and performs is also subject to many variables including temperature, humidity, pressure, the length of time the memory foam is compressed (it gets softer with time), body type, and sleeping style. Because there are so many variables with memory foam mattresses when you are ordering online the confidence you have that the mattress will be suitable for you (based on more detailed conversations with the retailer or manufacturer) and the options you have available after a purchase may be an important part of your personal value equation if you make a choice that is less than ideal.
I would be very cautious with all of the choices you mentioned.
Phoenix