Hi JustinP,
Normally back pain is a symptom of primary support (the support layers) that aren’t firm enough or comfort layers that are too thick but since your mattress only really has a single layer (with an inch of quilting foam or material on each side) and is firm … the odds are good that your pain is coming more from twisting to get away from pressure issues which means that the comfort layers probably aren’t thick enough. This could be compounded by having a mattress which only has 6" of latex which would be on the thin side for someone of your weight and it may be firming up too quickly under your greater weight (it may not have the range of response that greater weights usually need).
I understand that the primary issue is the back pain but it would also be helpful to know how it “feels” to you in your “normal” sleeping positions (and your girlfriend) in terms of softness/firmness and pressure relief and what your normal sleeping positions are (if you have a mattress that allows you to sleep in the way you usually do).
The first thing I would do though to make sure it really is an issue of the comfort layers is to check the box spring under the mattress to see if it has weakened or softened under weight over the years. You can test this by pressing on it firmly with your knees in different areas (especially the center which supports much or your weight) to make sure there are no soft spots or better yet to test your mattress on the floor for several nights to see if this makes any difference (and hopefully provide a description of how it changes your experience on the mattress for better or worse).
If your mattress is Dunlop … it may also have a slightly firmer and softer side so I would also flip and rotate it to see what differences there are (if any) on each side of the mattress.
If the issues remain and are not connected to the box spring (or the firmer and softer side of Dunlop) … then the next step would be to decide on the material you prefer as a topper. The choice between memory foam and latex (or other materials) is one of personal preference but they are very different from each other so I would make sure that you really did prefer one over the other. While they can provide similar levels of pressure relief … memory foam with your weight would likely be less durable and they would also have a very different feel (memory foam is a slow response material while latex and polyfoam are fast response materials). If you do decide to go with memory foam … I would make sure that it was at least 5 lbs density because of potential durability issues with your weight combined with lower density memory foams.
I would also talk with Hawn because they may know the details of the mattress you have and have some good suggestions as well that could be helpful. Knowing what you have would be helpful as a reference point in knowing what to add. They may even have a similar mattress and some toppers that you could test.
While you didn’t mention how your girlfriend feels on the mattress, with a lighter weight side sleeper I would suspect that she may also be feeling the mattress is too firm for good pressure relief? Knowing how she felt on what you have (and on her preferences between memory foam and latex or any other topper materials) would also affect your topper choice and help you find a balance that would have better odds of being suitable for both of you.
There are some good sources in post #4 here for toppers that would be within your budget.
The first step though is to confirm and decide on the material you prefer (and you could test both in local stores) and then to make the most suitable choice in that material.
My sense is that 3" would be a good thickness (assuming that your further feedback also points in this direction) because there is little in the way of comfort layers in your current mattress so there would be less risk in going with something thicker than I usually suggest be added to a mattress that already has a thicker existing comfort layer.
I may be able to make a few more suggestions or at least guesses that are a little more specific with a little more information and a more detailed description of what you are feeling on the mattress that can fill in some of the gaps and unknowns involved in making your best choice.
Phoenix