Hi Dave,

I am a little confused as to why at less than 11 st each, you would opt for a firm mattress. Spring tensions are dictated by body weight and for someone of under 10st, I would suggest soft springs, 10st - 17st medium and over 17 st firm. This is because the springs need your body to operate correctly and give the right level of support. The feel of the mattress is entirely different and is provided by the upholstery layers.
 

Using the above, I would suggest that you opt for a medium spring tension, assuming that neither of you are less than 10st, although this advice would need to be revised if you confirmed otherwise in terms of body weight. Please view the Artisan Naturals, Artisan Bespoke 004 and Artisan Bespoke. All of these products can be used with the type of base you refer to, although I would advise placing some form of barrier between the mattress and the base, usually in the form of thin (3-5mm) MDF to provide the mattress with a uniform surface upon which to sit.

Once you have viewed the above products, please call our office to obtain more specific advice as to suitability. Kind regards.