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More often than not, your amp's output is more than your ears can handle.  To maintain the tone of a cranked amp with saturated tubes and output transformers, a device called an attenuator is placed between the output of your amp and the speaker.  Weber offers a variety of attenuators for different applications.

A note about the power level: You should select an attenuator that can handle at least as much power as your amp puts out.* (see FAQ)

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.

Please read and consider the following prior to ordering an attenuator:
(The following text is reprinted with permission from Eurotubes http://www.eurotubes.com/euro-y.htm)

Here are some facts to consider when using an attenuator. Attenuators do work. They are not the cause of transformer failures. An attenuator can only simulate two out of the four components that are involved when you crank up an amp. The four components are #1 preamp tube distortion, #2 power tube distortion, #3 speaker distortion and #4 the physical movement of air that your speakers produce at high volumes. So the two components missing are speaker distortion and the physical movement of air and these are huge components! So if you’re playing at moderate volumes and just want to tame the amp down to get a bit of power tube breakup then an attenuator is a great tool, but do not expect to be able to dime your amp and then use any attenuator on the market to get the exact same sound from your amp at whisper quiet volumes. You can't get there from here. You would be better off with a one watt amp.

So, now some answers. Catastrophic power tube failure will cause transformer and screen grid resistor failures. The reason you will read and hear horror stories from un-knowledgeable attenuator users is because they do not understand how attenuators work and the stress they put on your tubes.  If in fact, a player chooses to run his amp cranked all the way on and attenuate the volume down to a whisper, then in most cases power tubes will only go a few months. Power tubes will die in one of two ways. The first and most pleasant is by simply fading and dying. The second and more popular way is to go out in a blaze of glory which is what I referred to above as a catastrophic failure which can and most often will, take out screen grid resistors.

Tubes sound great when they are really working hard but the harder you work them the faster they wear out. If you understand this and either use the attenuator sparingly or change your tubes regularly then your amp will not suffer damage.

If you are going to dime your amp out and attenuate it down to very quiet volumes then we recommend using an attenuator that is rated for twice the power of your amp's rated output.

If you are simply going to dial an amp up to where it starts to breakup in the clean channel and then knock it down a few dB so the club owner and the sound man are happy, then using a 50 watt attenuator for a 50 watt amp will be ok.

Understanding Attenuators
An article by Michael Farnsworth
This is a wonderful article written by one of our customers, which explains attenuators in even more depth.
CLICK HERE TO READ IT