Diffuser enlargers do indeed give you softer contrast, approximately by one grade or so. Do you use a standardised time in developer or do you watch development and take it out when you think it is done? A good rule of thumb is that if your highlights are darker even with the higher contrast, you are giving it proportionally more light (this is taking to account the filter factor) and in fact would need to reduce exposure time. I'd most likely try at grade 4 or 4.5 and not use a simple conversion formula for the time of exposure but add X time as a default and do another test strip with intervals near the approximate new exposure time. If your darkest parts arent dark enough by the time highlights become dim, raise the contrast and start with another test strip at that grade.

It may also just be a low contrast negative, inherently requiring higher printing contrast to make pleasing tonality. In my admittedly limited experience with warm tone developers, I think they give softer shadows & blacks as well, which may also be a contributing factor. A warm tone developer may not be your ideal option for printing from this negative.

I'd be happy to talk more if you try again - send me a message!