Hi LikeToHike,
You could test the box spring by firmly compressing it by hand in all areas (with your hands or knees) to make sure that the firmness of the surface is still solid and even. Even if it is though … it will still change the feel of the mattress compared to using a firmer no flex foundation. Whether this is “good” or “bad” depends on whether the sleeping system as a whole provides you with the pressure relief and support you need. It would also depend on how much you could feel the springs through the mattress. Most foam mattresses are designed to be used with a rigid non flexing foundation but in some cases with thinner mattresses they are designed to be used with an innerspring or “active” foundation to provide more contouring and pressure relief because the thinner mattress by itself may not be enough. If you purchase a mattress for example that is designed to be used with a firm non flexing foundation and put it on an innerspring … it may compress too much under your heavier areas and put you out of alignment (and could cause a sore lower back). Putting the same mattress on a firm non flexing foundation would result in better alignment in this case.
Generally the manufacturers recommendation should be followed because they are the best source to know the type of foundation that works best with their mattress. The most common and usually “best” choice though for this type of mattress would be a firm non flexing slatted or “grid” foundation and as I mentioned putting it on an “active” box spring would change it’s performance and allow the heavier areas of your body to sink down more. How much this affected you would depend on your height/weight/body shape and sleeping positions and how they interacted with the mattress. It’s generally “safer” to use a rigid slatted or grid foundation though.
Some of the lower cost foundation options or this type are in the posts that are linked in post #60 here.
There are some people who believe this is OK but my personal preference is towards a non flexing slatted or grid surface because it allows the mattress to breathe and ventilate better and is “safer” in terms of trapping moisture in between the plywood and the underside of the mattress (which can encourage the growth of mold and mildew and other “undesireables”).
@ garaet
The information in your last post is very non specific and vague and isn’t really alll that helpful. It’s typical of the type of information that is found all over the internet and is like saying “if a mattress isn’t comfortable you won’t sleep very well”. Innersprings and box springs are not the same thing although they can both contain various types of springs. Box springs are used under a mattress as either a shock absorber (to prevent damage to the innerspring) or to change the performance of a mattress. Innersprings are part of the mattress itself and are used primarily as the support system of the mattress (although different designs can affect both pressure relief and alignment to different degrees).
If you read the overviews in the mattresses section of the site you will find a lot more specific and accurate information about innersprings and the main functions of a mattresses (pressure relief and alignment) and the different ways to achieve them for each type of person.
Hope this helps
Phoenix