Artisan 1500 mattress, topper or protector?

Hello
We bought an Artisan 1500 superking ziplock mattress from you last year which is wonderfully comfortable, my husband & I have both got rid of any backpain since using it. We have always used it with a thick wool filled topper which we are finding annoying due to the shift in filling. Is it necessary to have a topper with the Artisan 1500 or would a topper suffice? If so, what do we look for when choosing a topper?