Hi Rjb38,
Welcome to the Mattress Forum! 
Iâm sorry to hear about your back issues. 
Unfortunately, I canât diagnose nor make recommendations as to what might fix your problems via an online forum, as there are entirely too many individual variables involved in each personâs own comfort preferences. Additionally, youâre not aware of exactly what you have regarding foam layers (we know that the latex is from Mountain Top, which means that it is Dunlop, but you donât have ILDs or densities). Iâll do my best to provide you some general guidelines which you may find helpful.
There is some information about the many different symptoms that people may experience on a mattress and some of the most common causes behind them in post #2 here that may be helpful. The most common reason for lower back pain is a mattress that has comfort layers that are too thick/soft or a support core that is too soft. In your situation you state that the 6" support core is firm, and then I believe you have another 3" firm layer and then a 3" soft layer. As you are heavier, my best guess would be that the uppermost plush layer is too soft for your personal preference. Youâre not specific on the exact combinations that you tested out with the wool and the memory foam (I donât know if those were placed upon the existing mattress or in place of the 3" soft layer), but placing memory foam on top of a mattress generally wouldnât make it feel âfirmerâ, as memory foam is generally a quite soft material. It may have caused you to âfeel throughâ to the deeper firm layers, which you experienced as a bit of a firmer feel.
I think your thoughts of replacing the soft layer with a firmer layer (medium) would be a good start. With the layers you have right now, have you attempted rearranging them so that the soft layer is in the middle with the firm 3" layer on top, or have you tried the mattress without the soft layer completely?
If the latex was continuous pour from Mountain Top, their ILD ratings are as follows:
Their 25% ILD ranges are âŚ
C0 9.5 - 12.5 ILD
C1 11.5 - 14.5 ILD
C2 17.5 - 20.5 ILD
C3 21.5 - 24.5 ILD
C4 29.5 - 32.5 ILD
Their 40% ILD ranges are
ILD Range
C0 14 - 20 ILD
C1 18 - 22 ILD
C2 27 - 33 ILD
C3 34 - 42 ILD
C4 44 - 54 ILD
The closest ILD approximations to Talalay for each of their different firmness levels for individual layers would be somewhere in the lower end of their 40% ILD range. Itâs possible the #2 and #4 were the C2 and C4 from this chart, so you can see the C3 would be between those two softnesses.
While I canât speak to how any specific topper will feel to you in combination with your mattress ⌠the Pure Green topper is 100% natural Dunlop latex which is certainly a high quality and durable material but if you canât test a mattress/topper combination in person then the only way to know whether any topper will be a good âmatchâ for you in terms of âcomfortâ firmness, and PPP will be based on your own personal experience when you sleep on it.
The choice between different types and blends of latex is a preference and budget choice more than a âbetter/worseâ choice. There is more about the different types and blends of latex in post #6 here and there is also more about some of the general differences between Dunlop and Talalay in post #7 here but again the most reliable way to know which type of topper or which type of latex you tend to prefer will be based on your own careful testing or personal sleeping experience. Talalay will tend to have a bit more of a âlivelyâ or Buoyant feel than Dunlop the continuous pour Dunlop from Mountain Top seems to fall in between these two), and at the same ILD a piece of Dunlop will âfirm up fasterâ than the similar ILD piece of Talalay.
If you do decide to purchase the Pure Green topper I would also check the Sleep On Latex because the price for the same topper may be lower (I canât get on amazon right now to compare prices â their site is experiencing some issues.) Also, Sleep on Latexâs exchange and return policy can be considered the same or better than Amazonâs.
Because Sleep On Latex protects itself against the code harvesters you can request the code in a Private Message or email. Alternatively you can always place your order over the phone (312.487.2865) and mention that you are a member of The Mattress Underground to receive your discount.
I wouldnât be as concerned with the brand of the latex pourer, as most latex is a high quality and durable material.
Overall, it sounds as if you are moving in a logical direction with slightly firmer upper comfort layers, but in the end the only way to tell would be through your own personal testing. And as heavier individuals tend to need/prefer thicker comfort layers, I think your thoughts of staying with the 3" layer are also good. Post #3 here has more information and suggestions about higher BMIs that is worth reading.
Phoenix