Slatted Bedstead Preparation and *Feel*

After spending a great deal of time trying out various mattresses, we're rather keen on the ViSpring Herald Superb in medium tension, however our budget of £1000 (for king size) doesn't stretch to this indulgence. As such, judging by your comfort chart and comments on the Herald Superb, we're likely to go for your Artisan 1500 with it's firmer feel and great price. This has led to two questions:
We currently have a bedstead with wide rigid slats offering little to no flex at all. We're going to board over this with some plywood we have, however we also have some left over carpet underlay from our recent flooring. It's good quality underlay, 10mm thick foam - would it be worth sticking this to the plywood to offer a slightly cushioned surface on which the mattress could rest? It will be wrapped in an old bed sheet regardless and I plan to put the plain plywood side onto the slats and the slightly padded side facing up to meet the mattress.
Secondly, when trying out mattresses, we got completely conflicting advice from different salesmen (both within the same store!). On suggested that putting a mattress on a solid slatted base is akin to having it directly on the floor and will make the mattress feel much firmer. The other suggested that putting a mattress on a solid slatted base is akin to having it directly on the floor and will make the mattress feel much softer. And I can see a degree of logic behind both suggestions! So, I wonder:
- will the mattress feel firmer, because it doesn't have the additional flex of a sprung base?
- or -
- will the mattress feel softer, because you sink further into it, rather than relying on the give in a sprung base?
Thanks for your help!