Cons: Underpriced.

Tell me more about it.

Sure.
That mainly has to do with the excursion factor and people comparing them to mid-fi headphones , I think the dt880s and 650s should be in the $700-1000 range , its not exactly a con but in a way it is , since people buy it and then compare it under-performing Mid-Fi headphones like - X1s , 598s , 1840s , 1540s , 1440s etc 

if you like HD650, you should check out LCD-2 or LCD-3

I think the prices of headphones should be reduced overall. There are definitely a lot of under-perfoming mid-fi headphones out there. I haven't heard the others you listed, but I did find the HD 598 an underperfomer. I guess it's a good idea to generally stick with the mid-fi trio.

@Valhallatier
 
I would but the Comfort of those headphones is a deal breaker for me 

@kman1211
 
Agreed , Classic Trio FTW   

I think that the Valhalla 2 is an awesome amp for the price and for the HD650, not perfect but damn good. It has excellent trble as well as nice lean bass that is obvious but not intrusive, with good spatial imaging (not so true with bad recordings or super complex songs), Here's some photos of my setup  http://imgur.com/a/7ISj3
 
I actually agree with you on the price point and grouping of this headphone with mid-fi headphones. Not too long ago this was one of the best headphones in the world, I don't think it's fair to compare it to headphones that were produced to fit their price point, I like going back in time and reading the Amazon reviews from 10 years ago. I'm not saying jack the price up to 2-3x it's current, but it should be treated as so much more than what it costs.

@icomrade

Great setup ! 

In what year DID the Sennheiser 650 first come out?  Should I wait awhile and hope Sennheiser brings out something new but a better sound? Thanks

@AtrafCreez

I don't think they will come out with anything new soon......Senn just revised the 650 packaging , that should be a indication.
As far as I know the 650 was launched in 2003 , hope that helps !