since there are still a lot of "Setting Clean Sense" threads popping up, I thought I could share my approach to set it. I think the "official" way of strumming very hard and looking at the input LED works, but I get that it leaves people in doubt if they have really set it correctly.
Here's how I do it:
1. Connect your KPA to your DAW
2. load a good RMS meter (peak metering won't help you, because the clean sounds are too dynamic)
3. Load any rig that you like and set the gain to the maximum value
4. Strum hard and memorize the value on the RMS meter
5. Turn the gain all the way down. Strum hard and set Clean Sense in a way that the RMS meter reads the same value.
You can also do the balancing by ear, but the RMS meter is probably the better choice.
Clean Sense (as ck stated) "takes care about the loudness of clean sounds while not altering distorted sounds"
So to set it right you should set it in a way, that the clean sounds are exactly as loud as the distorted sounds. You could do this by using two different rigs (one clean and one distorted) but turning the gain knob makes more sense to me.