I had some responses over at Linus' forum, but I would like to get some more input before I buy. I'm curious if I should get either the Sennheiser HD 600 or HD 650. I have heard from some people that they are the same, and some that they are different. I will be using these with an O2+ODAC combo from Mayflower. I will also be listening to House, Rock, Electonic, Metal, and some Hip-Hop. I don't really care about gaming, because it really won't make my experience that much better in my opinion. I am also wondering about the differences between them not just limited to sound quality. Is one more sturdy and well built than the other? What about comfort? Warranty? I will most likely be buying these from Accessoryjack.com, they seem to have the best prices according to Google Shopping.
They do sound different when compared. I don't know if frequency response means anything to you but in this case I think it's pretty representative.
The HD650 is a newer design, though they are both made of the same materials, and very similar build quality (=good). The grills on the HD650 are made of a tougher metal mesh though so they can take more hits without getting dents. Anyways, you would have to try hard to break any of them. Comfort is pretty equal.
As for the sound - the HD650 is: darker, warmer, more laid back, more bass, slower. The HD600 is: more neutral, faster, more forward (peak in 3-4 KHz region), lighter bass.
The HD600 can actually sound more detailed thanks to the forward treble, but on the same time introduce a slight harshness that some people don't like.
You mentioned "House, Rock, Electronic, Metal, and some Hip-Hop" as your main choices of music. My choice there would be the HD650, HD600 might be preferred by some for e.g. prog rock and extreme metal, but for house and electronic music the HD650 wins while still being a solid performer in other genres.
Note that both are excellent headphones though.
I suggest that you take a look at both the HP100 (which I have, and love the bass quality/quantity of and think that it is enough for any genre) or the Mr. Speaker's Mad Dogs. Both of those headphones are a better value than the hd600/650, so that should be somewhere to start. Reviews of both of those headphone can be found on head-fi, so check there. Do note that the review of the HP100 by ClieOS (whose reviews I generally love) was done with a pre-production pair of HP100 and I don't think that what he had sounds the same as the final product. There is much more bass than what he described. It seems to me that Soundmagic saw what he said about the headphones and made adjustments accordingly before releasing them.
And as far as amps go, check out the Schiit Magni and Modi. They are considered to be a great value set, though I personally find the E17 to be more than enough amplification for my HP100. The Mad Dogs, hd600, and hd6580 all require more amplification though, so keep that in mind. Something like the O2 or the Magni should be plenty though.