Since this keeps cropping up everywhere, I thought it deserved its own thread.
I have some experience with an old Tom Scholz Power Soak with a '69 Super Reverb, and thought it was pretty cool until I talked to my amp tech about these things. His comments aside , I haven't yet tried one of the Weber MASS units, but I keep hearing good things about them, and definitely want to try one with my Sovtek head.
I found a couple lengthy write-ups on the Weber MASS and the THD Hotplate:
Weber MASS Attenuator
THD Hot Plate
...and another I've not heard of (new, apparently):
New Product Preview! "The Ultimate Attenuator"
I can only speak for my experiences with the THD, but I'm a big fan of the thing. With Bright and Deep switches engaged, even -16db of attenuation is still pretty transparent, and with my Rectoverb it was the difference between an amp that was pretty toneful in safe bedroom jamming levels, and something thin and fizzy until it was WAY too loud for home use.
For the Roadster, I'm running it at -8db most of the time, and really if there's any real tonal change, I'm having a hard time hearing it.