Hi chopeb,
It’s certainly possible that you are still adjusting to a new mattress and your body can take some time to change from its “learned alignment” that comes from sleeping on an old mattress (see post #7 here).
If your back pain isn’t from a normal adjustment period then the most common cause of lower back pain is from a mattress that is too soft and which “allows” the heavier parts of your body to sink down to far before “reaching” the firmer support layers which can put the lumbar spine out of alignment. There is more about some of the more common “symptoms” on a mattress in post #2 here and the posts it links to.
Finally it’s also important to remember that a mattress can’t “fix” a pre-existing back issue that isn’t connected to the mattress itself so there may be no “perfect mattress” that fixes things until the back issue itself is healed (although it’s still important to have a mattress that keeps you in good alignment because it can still be an improvement and will be much less likely to aggravate any back issues). A pillow under the knees when you are sleeping on your back (or using an adjustable bed with the legs slightly elevated) is well known to relieve some of the stress and compression on the lumbar spine and is a good suggestion for any mattress for back sleepers. Sleeping with the knees slightly bent (on your back or side) is a more natural and less stressful sleeping position for the body.
If your continuing experience indicates that the Elegance is likely too soft for you then the Essence would be the next most logical choice and has thinner comfort layers that will keep you closer to the firmer support layers of the mattress so your pelvis doesn’t sink into the mattress quite as far.
Phoenix