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I consider soft proof broken in Darktable , while using filmic. Although this has to do with bright and saturated colors , not the overal gradation and tone.
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your monitor is calibrated, but also at around 100 nits ? A too bright monitor is often a beginner mistake in consistent prints. Print media has no backlight:).
When you soft proof, do you use an ICC file from the print lab ? It could be that if you are just soft proofing to sRGB, their prints might just be darker then sRGB (lots of ink takes away the white ).
On some at-home consumer printers i regularly just apply a shadow boost just before saving and printing . 'if it looks good on my screen, make it brighter and print ’ was my rule :).