It's safe in some amps, but not so safe in others. You would probably need to remove the 2 outter tubes in a 4 pt amp. You would also need to half the impedance. If the amp has an impedance selector and you run a 16ohm cab, you would need to set the amp to 8ohm. Also, it's not going to make as big of a difference as you might think. The difference between 100 watts and 50 watts is roughly 3dB's. This is still rediculously loud. If anything, you might get more power tube break up earlier and loose some low end. Your best bet is to get an attenuator. THD and Weber make some nice attenuators that work exceptionaly well. I'd suggest checking them out before you start pulling tubes. I'm sure Lats can give you a more detailed explanation though.