The CleanAmp amplifies the original audio of the console. It also removes 100% of all noise when the volume wheel is down.
The Dehum/Dehiss kits remove hums (lows) and hiss (highs) from the audio at all volume levels.
With the CleanAmp installed you can turn the volume wheel down and it will act as if you have no speaker even connected, it physically disconnects the speaker so you get no noise at all when the volume is off, for pure silence.
The Dehum/Dehiss kit removes all the audible "noise" you can hear on the consoles when the volume is up. This is the most recommended upgrade as it really improves audio quality.