The CleanAmp amplifies the original audio of the console (The white v1.1 and newer have PureSilence built in, no need to purchase separately).
The PureSilence removes 100% of all noise when the volume wheel is down. Works for the GBA only at the moment.
The Dehum kits remove hums (lows) and hiss (highs) from the audio at all volume levels.
All 3 work together. If you install just the CleanAmp you will hear noticeable humming and hissing at all levels, even when the volume is completely off.
If you install the CleanAmp + PureSilence then when you turn the volume wheel down 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 voume is off, for pure silence.
The Dehum 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.