We suffer from lower back pain, is the Luxury suitable for us?

I have read most of your website but want to check before making such a big purchase. My partner and I weight between 10 and 11 stone. I am 5'8 and he is 5'11. We both sleep on our backs and sides. I have lower back pain and stiffness caused by poor support for our current mattress I think. We both like a softer 'sink in' matters feel.
I plan to buy a king size Artisan Luxury mattress with an origins platform base. Do you think this will suit our needs ot should we think about a firmer feel mattress?