This may sound silly, but it sounds like you might be describing the discovery of back problems you weren't aware you had.
If continuous supportive (not excessive) pressure on your lower back causes pain, you might want to try talking to your doctor and springing for a series of good therapeutic massages of your lower back and glutes and seeing if it improves.
You're probably experiencing pain because there is either inflammation in that area or tight / knotted muscles in that area or your glutes. The Touareg doesn't provide any support there, so there's no pressure. Instead, your muscles stay tensed up and your body and mind have adapted to keep you from feeling what's wrong there. When the Volvo applies additional pressure you are not used to, even if that is properly supportive pressure, it may cause you pain.
You could just try to adjust the seat and ignore it, but eventually you may end up with more serious problems. For example, I didn't know I had these kinds of problems in my upper back until the muscles literally pulled my spine crooked, leaving me in a great deal of pain and discomfort and causing a chain reaction of other muscle strains. With the lower back, you could end up with hip problems or the sciatic nerve could become impinged resulting in sciatica.
Just suggesting you look at this from another angle - if the problem is actually with your body and not with your seat, you may do yourself a disservice by just trying to adjust the seat to compensate.
That's not to say there isn't great value in comfort and you should stop trying to adjust your seat. Our old tempurpedic (which we got when I started realizing I had back problems) let me feel everything that was wrong with my back when I laid on it because of the even pressure. We traded it for a super soft cushy pillow top and I am sleeping much better now and will never go back.

I can't imagine the driving distraction that pain might cause!