And @RawConvert replied just as I was about to start typing
Anyway, what @RawConvert said seems right based on what I know about RawTherapee soft proofing, which is not much.
The question I actually wanted to answer is whether soft proofing from sRGB or ECI-RGB to ISOcoated_v2_300_eci would give different results, assuming the output profile was used as the source profile for soft proofing. To answer this question requires having the test chart.
But in RT apparently the output profile isn't used as the source profile for soft proofing, which seems a bit odd but perhaps useful in the context of overall RT functioning.
Then I was going to ask the following questions about RT soft proofing, which I'll go ahead and ask:
What source profile does RawTherapee actually use for soft proofing to a user-chosen soft proofing profile?
It might be using CIELAB?
Or the user's chosen RGB working space?
Or it might be using the user's chosen output profile? (apparently not)
Or the image input profile?
And does RT use clipped or unclipped conversions from the image's original input profile (or the camera raw profile for raw files) to CIELAB, and/or to the user's chosen RGB working space, and/or to the output color space, and/or to the soft proofing space if the soft proofing space actually supports unclipped conversions?
There seem to me to be many "moving parts and possible paths" in RT soft proofing. I would love to see an explanation of the actual conversion paths for RT soft proofing, taking into account the issue of "clipped vs unclipped" ICC profile conversions.