For what it's worth, I export from SPP as neutral as possible (including backing off the sharpening from zero quite considerably) as 16-bit TIFF ProPhoto. In RawTherapee, I select ProPhoto working space (RT's default) and 'use embedded profile if possible'.
It's a pain going from one app to another but, for my SD9/SD10 X3Fs, the results are way superior to my alternative of ACR 5.4/ PSE 6.
cheers,