Hi Paisley,
I’m not quite sure what you mean here. A foundation to support the mattress is generally above a bedframe not under it (it goes right under the mattress). the type of foundation that would be best would generally depend on the type of mattress you purchase but if the mattress is a foam mattress then it’s generally best to use a firm non flexing slatted foundation or platform bed with gaps between the slats that are narrow enough to prevent the foam from sinking into the gaps over time. A foundation is meant to provide even and non flexing support under a foam mattress rather than be part of its comfort. There is more information about foundations in post #1 here and the posts it links to (in the second paragraph).
With an innerspring mattress the type of foundation or box spring recommended by the manufacturer is generally best.
Based on your feedback it seems to me that there may be soft spots in your mattress which are allowing the heavier parts of your body to sink down too far and if this is the case then any toppers above the mattress will generally just follow the soft spots underneath them (see post #4 here).
It may be worth trying your foam toppers on the floor (or on plywood over your mattress as jrc mentioned) to see what difference it makes when the toppers are on an evenly supportive surface compared to the same toppers over your mattress.
I would also keep in mind that there are several types of alignment. One is horizontal alignment of the spine so that it can maintain its natural curves but there is also rotational alignment where part of your body is twisted and there is also the natural alignment of your joints which can also create joint pain if they are resting near the edge of their normal range of flex.
Sometimes a more specific description of the type of pain or discomfort you are experiencing, the sleeping positions you tend to feel them the most, and the exact location of the pain can be a useful pointer as well.
Phoenix