I also use the Weber Mass products (Stereo Mass and Standard Mass units) and they are great IMO. The set up you are describing would be cool...but I would go for the 50 watt units based on what I read at the Weber site...
"A note about the power level:
You should select an attenuator that can handle at least as much power as your amp puts out.
Amplifiers are generally designed to operate clean, and most manufacturers specify the output power as clean power. When you run the amp full blast, the output devices actually become more efficient because distortion is not considered. Under those conditions, the amp may be capable of putting out as much as 50% more power than its rated clean power. Be aware of that when choosing an attenuator, because the idea of using an attenuator is to run the amp full blast into power tube distortion and then controlling the level going to the speaker. If you are going to dime a 50 watt amp, you should choose a 100 watt attenuator."
http://www.tedweber.com/
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