Using Audio-Technica M50 as speakers, will this break my headphones?

Break your headphones? No. It'll just sound like tinny garbage, worse than $10 DealExtreme portable speakers. It might make them wear out a little faster, but still on the order of years and years I would assume.

Depending on how loud it is:  Yeah, you actually could do some damage.  Really though,  it's just going to sound terrible.

It will sound like s*** though. Usually the max power a headphone can handle is right before you hear distortion, usually by maxed out excursion(Xmax).  

Amazing that what a sick mind will do to a fine pair of HPs.  Some el-cheapo Coby from the flea market will do a better job if one is so broke to buy a decent pair of speakers.

That's said, now I understand why there are two many decent , pretty and divorced women.

Break your $130 headphones because you can't go out to the $1 store, or go out to the $1 store.

Your choice.

Btw, if your can't can't take that much energy like a T50RP can take a lighting bolt, don't do it.

Like everyone else said, don't do it.

You could buy cheap computer speakers and the output would be better. Headphones should stay on your head :)

spend 20 bucks on a portable speaker. like the fiio one, whatever its called.

why would you do that to your awesome headphones???