The Weber MASS (Mobility Analog System Simulator) is an attenuator designed for use with electric guitar amplifiers. It should be used with tube based amplifiers, as it allows you to use a tube amp at full power without needing to have the volume at full blast. As you can imagine, this is useful in all sorts of different situations. Having an attenuator like the MASS is primarily something that will be used in the studio, but is definitely helpful for a live show as well. I however have only used it in the recording studio in tandem with a Fender amplifier head. Nothing can beat the sound of a Fender tube amp when cranked at full power, so having this in the line to do so while not having to have the volume up all the way is a beautiful thing. The version of the MASS that I used was a different one than in the picture above, and it was the 100 watt model. The MASS comes in a few different watt sizes, including 50 watt and 25 watts for using this type of thing with a smaller amp. Beyond the wattage, I don't think there is a difference between the modes. As far as parameters go, the MASS has knobs for bass, middle, treble, and volume, as well as those for impedance and speaker volume. It also has switches for tone bypass, a treble booster, and to choose between either a high or low range. AS you can probably tell, the MASS allows for a good amount of control over your sound beyond what your amplifier will give you, which is always nice. It goes right in between your amp head and your speaker or can be used with a combo amp as well. I've only really used one other type of attenuator like this, which is the more expensive THD Hot Plate (the 8 ohms version). Although the THD Hot Plate is a great product as well, I'd recommend going with the Weber MASS based on price alone, as they really do the same thing and there is a significant difference in price. I don't even think that the THD Hot Plate is overpriced, I just think you're getting a great deal with the MASS...