Re: powerbrakes / hot plates
Yes, they are safe. This type of device is generally called an attenuator. It's connected between your amp and your speaker(s). The only concern is that you should get one that is matched with your amp's impedence (4 ohm, 8 ohm, or whatever) and is rated to handle at least your amp's RMS wattage (otherwise, the attenuator may overheat).
Yes, Hot Plates have received more favourable reviews than the Marshall Power Brake. You should also look into Weber attenuators, which also provide good sound. You should know, however, that with most attenuators, when you greatly attenuate the amp's signal (for example, to conversation volume levels), your tone will suffer, typically by showing reduced presence/treble.
Hope this helps!