New mattress equivalent to the John Lewis 6000 spring king size

We have a split tension Harrison 4000 spring bed (firm/medium). Wife is 12 stone and i am 14 stone, but have they damaged right shoulder which occasionally causes me pain and neck ache in my sleep.
I think I need a slightly softer mattress, but do prefer firm to soft, where's my wife would like medium because of her damaged back (ribs). We've tested the John Lewis Harrison 6000 springs in a firm which seem to be soft yet supportive, but were intrigued by your beds.
What would be the equivalent or better in your range with a single tension if possible?