Divan base for Vi-spring Classic Bedstead mattress

Can you please tell me how your Artisan base compares with the Vi-spring divan bases ie the standard and prestige versions? I have a classic bedstead mattress on a solid slat base and am thinking that the base is making the mattress (medium tension) too firm for me. I weigh 9.5 stone and realise I should have probably gone for a soft mattress ( I was not aware of your company at that time!). So I am hoping to find a way of making the mattress feel a bit more "giving" or "responsive" as I have back problems. Could you please tell me how many pocket springs are in the artisan base and whether it is possible to upgrade the 'fillings/coverings' to make it as comfortable as possible?
Many thanks