A: The two are completely different. Multi-Soak is an attenuator that is channel assignable. Multi-Watt allows the player to assign an output wattage to a given channel. For example, each channel on a TransAtlantic 30 has three wattage settings: 40, 30, and 15.
While choosing the different wattage will lower the volume, it will also change the gain, headroom, and feel of the amp as the actual topology of the power section is changing. Multi-Watt is a more effective way of adopting different styles of music by faithfully recreating the actual power section (using 4 tubes, 2 tubes, or 1 tube) to get the desired tone.