Mattress Forum

My wife and I decided, after going and laying on several mattresses, to go with a latex mattress. In a store, we laid on a latex mattress that we were told was 6" 32 ILD latex with 1" foam (softer, non-latex) on each side. We really, really liked this matress. What we ended up doing was buying a 6" 32 ILD latex mattress from sleeplikeabear.com (because it was much cheaper than what we tested in the store). Our thought was to start with this and then fine tune the mattress as needed. This mattress arrived yesterday. 

First, let me say that last night I slept better than I have in ages and this is a really good sign. However, both my wife and I wish it was a little firmer. The 32 ILD latex mattress we laid on in the store felt firmer and more supportive of our hips than this 32 ILD latex mattress we got from sleeplikeabear.com. I am pretty sure both came from Latex International, so I am not sure why the one we have feels noticeably softer than the one we laid on. 

On to my question. How can we make this mattress feel firmer and more supportive of our hips? Right now, the mattress is on top of our old foundation, which has wood slats. If we put a piece of plywood on top of this, would that make the mattress feel firmer and support our hips better? 

Or, if we bought a 2" or 3" piece of 50 ILD latex and put it underneath or on top would that support our hips better? 

Or, would it be better to try to exchange it for a firmer latex mattress altogether and go from there?

Any other suggestions?

Thanks in advance!