Ok, I was trying to give it more time before responding, but so far with the same EQ settings and equipment, the Yamaha Pro-500's have not only more bass, but more clarity in the mids and treble. Bass quality is about the same whereas the HP-500's a slight boost in mid-bass. With EQ settings flat, the M100's have more bass. The song I'm currently using is Kendrick Lamar - B@$ch don't kill my vibe for it's powerfull 808 drum decay, vocals and high hats. The Nuforce HP-800 sounds very interesting for the price.
It's interesting to note that the PRO 500 sounds at least as good as the M-100. It's a shame that they aren't very popular on Head-Fi.
Still, for the moment, I'm going to pass on the PRO 500 - I'm not really a huge fan of its design.