Hi EZ4HZ,
You can see some of my comments about the “wire grid” types of support surfaces in post #10 here.
I would also agree with Jeff that the ones that have wires that are further apart may not provide a support surface that has enough surface area to provide an evenly supportive surface for some types of mattresses including mattresses that use some types of innersprings or latex support cores because the support system in the mattress can sag into the gaps in between the wires over time.
I would also be cautious about using plywood or any solid surface under a mattress because it doesn’t provide any ventilation under the mattress (see post #10 here) and would lean towards the suggestions in the previous post I linked.
There is also more about the different types of support systems (foundations, platform beds etc) that are generally suitable for different types of mattresses in the foundation post here. The KD foundations that have wooden slats that aren’t too far apart for the type of mattress you are considering can make good choices because they have more surface contact/support area than many of the wire grid type of support systems. When you are dealing with a knowledgeable and experienced manufacturer such as Beloit though I would trust their suggestions about the type of support systems or foundations that would be most suitable for the mattresses they make (and which ones would meet their warranty criteria) because they will be the most reliable source of information about their specific mattresses.
Phoenix