holyyy flying monkey hellll.... that is legit $2,550 worth of headphones you are holding onto right now.
how are you not happy w/ the TH900 + AKG K550 + HD600??? those are 3 amazingggg headphones. The $1,500 TH900 especially, that is like my dream headphones! ...and you sound so nonchalant & unimpressed by it.
how have you not found your end-game headphones already? isn't the TH900 considered to be one of the best closed headphones out there (along w/ the new Audeze LCD-XC and Beyer's T5p)? are you looking for a portable pair of headphones or something?
I've never tried the HD650, but I heard the HE400 sounds semi-similar w/ more bass & more treble. If Momentums have too much bass for you, you can probably get enough bass from open headphones. You can also consider the Audeze LCD series. They even recently made a closed pair, LCD-XC. If you are just going for closed, perhaps consider Alpha or Mad Dogs/Paradox. All you are missing from your expensive headphone collection is a pair of orthos lol.