The NaNs related issue should be solved if you download from github the latest version (not the latest official released, but the developers version).
Regarding StaMPS results, if you plan to go to a GIS system it should be immediate or simpler to export in csv file and then import in the GIS and from there, if desired do the conversion to tiff… That is what I do when I want to work with the results into GIS.
I hope this helps