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Simple test:
Instrument rack, a synth in a single chain. The key range is set up to go all the way from the left (C-2) up to C5. Fade range is set at full volume across the whole range. I trigger a simple saw wave with the C5 note, and I get a peak reading of rougly -1.8dB.
Now, I grab the fade range indicator from the right (C5 position) and drag it all the way to the left. The result is a ramp spanning seven octaves, with the extremely quiet end right at C5.
I trigger C5 again, and the result should be very quiet, barely audible. However, the peak reading is around -4.3dB.
Summing up: the quietest note, even in a very long fade ramp, is merely around 3.5 dB quieter than the most audible note! In this state, the key range fades are useless for actual crossfading between layers.
Edit: The chain selector fade ranges work like they should, but the key fade ranges described above don't.