Hi asleeper,
I think the numbers you mentioned are the density of the latex because they wouldn’t be the ILD or firmness (you won’t find latex with ILD’s that high). If the layers are 100% natural Dunlop … you can see the approximate ILD’s for different densities in post #2 here.
Changing the bottom layer would have much less effect on the “comfort” and pressure relief of the mattress than adding a topper and it’s quite likely that the 2" 75 ILD comfort layer may be too thin or firm for you over a very firm support core. I would tend to add a topper to improve comfort/pressure relief rather than changing the support layers which have more to do with support/alignment than the comfort and pressure relief of the mattress.
Post #2 here and the topper guidelines it links to have some suggestions that can help you use your own sleeping experience as a reference point to make the best possible topper choice with the highest odds of success.
Phoenix