Hi zeshef.
And welcome to our Mattress Forum! 
With the 4th of July “busyness”, I thought I’d chime in with a few suggestions that you may find useful. I am sure that APM will have some suggestions for you soon, but you can check on a few things before going a little deeper into what you might need to address the pains that you and your wife are experiencing.
I’m 200 lbs 5’8" and my wife is 140 lbs 5’6" and both of us have severe back pain that wakes us up in the middle of the night sometimes and sore back every morning.
Sorry to hear that you started experiencing back pains. Just to check all your basis and even though you say that the foundation you built has a “ZERO bend” you may still wish to check its behavior when it is under load with the weight of the mattress and the people on it….especially with a King size bed I’d make sure that the center support beam is sturdy and has a couple of legs underneath. If you have any doubt that some sagging is happening “under the hood” I’d test this by putting your mattress on the floor to see if it makes any difference. If it does then it’s possible that your support system could be part of the problem as well. Sometimes it is also a combination of factors but it is important that you assess with changing only one variable at a time and see what results you get.
Another thing you may wish to check upon is the pillow you are using and how high/low it is in all your sleeping positions. A suitable pillow is an essential part of good alignment for the head and neck and upper body because the gap between the head and the mattress and the curve of the cervical spine needs to be supported just like all other parts of the spine. Like mattresses … there are certain “needs” that depend on body type and sleeping positions but with pillows, personal preferences play a more important role because the face is much more sensitive to textures, temperature, smells, and other more subjective “feel” based properties of a pillow. There is more about choosing pillows in the pillow thread here and the other topics and sources of information that it links to that may be helpful.
The two main functions of every mattress are to keep your spine and joints inside the range of their neutral alignment in all your sleeping positions and to relieve pressure by redistributing your weight across the surface of the mattress which is what you probably are referring to. Usually waking up with lower back pains during the night or in the morning points to an unsupportive mattress, which is confirmed by your experiments with the softer topper added and used in different layering configurations. While you are not mentioning your primary sleeping positions it may be that a 6" comfort layer may be too thick and/or too soft for your alignment needs with allowing your body to sag in a “hammock position” (depending on your body type and weight concentration in different areas of the body it seems that to a certain extent your hip/pelvis area may sink in too much). Have you tried removing the top Medium layer completely to sleep on M, F, XF configuration? If yes what were the results? At your BMI of 30+ Kg/m[sup]2[/sup] and depending on your sleeping position you may wish to experiment with removing some of the comfort layers to see if this makes a difference and to see if the pains diminish and to better support. If the comfort pressure/relief becomes a problem then later you can always fine tune it with adding more plushness for comfort.
Heavier body types will tend to sink into a mattress more deeply than lighter body types so their concern is that with a softer mattress (with thick/soft comfort layers over the support core) that the heavier parts of his body will sag and sink down too far before being “stopped” by the firmer support core and he will be out of alignment. Some schematics here and here will help visualize this. Both can lead to uneven support where the spine is out of alignment. The goal is to have comfort layers that are “just enough” and no more in terms of thickness and softness to provide good pressure relief and fill in the gaps in the sleeping profile and support layers that are firm enough to “stop” the heavier parts of the body from sinking down too far.
Having a high BMI presents special challenges and generally requires firmer materials (in the support layers especially). This could be firmer latex or innersprings (the type of support component would be a personal preference and in the right design either could be suitable) or even a zoned construction. The same overall guidelines apply with higher weights though that PPP (Posture and alignment, Pressure relief, and Personal preferences) along with using high-quality durable materials that will maintain their feel and performance for longer periods of time are the way to make the best choices. Heavier people, in general, will need firmer and thicker comfort layers and firmer support layers than those who are lighter and because no materials will last as long with much higher weights the quality and durability of the materials and components is even more important than normal. I wouldn’t “rule out” any types of mattress and base your choices on your own personal testing.Post #3 here has more information and suggestions about heavier weights that is worth reading.
Hotel mattresses, while not a single “type” of mattress, do tend to be in the general category of what many would call “medium firm” which means that they have a medium plush layer over a firmer support core. This “feel” tends towards the average preference of a wide cross-section of the population. A more recent trend has seen hotels choose items that are a bit firmer feeling and then customizing their mattresses with various “top of bed” products such as plush mattress pads and polyfill toppers. These are easier to replace and launder, and offer a more cost-effective solution to add a bit of plushness to a mattress, and the items closest to your skin – the sheets and pillows – can have a dramatic impact on the overall impression of comfort made by a hotel mattress.
Some people have longer adjustment periods than other and it may be connected with how "engraved " certain sleeping patterns became overtime and how your body made up for any issues over time. While some people react straight away some need a few weeks or so.
Many of your answers will come with a bit more experimenting as it seems to me that you are missing some important pieces from your mattress pains puzzle.
Phoenix
