After a night of listening to them, I would have to agree, they do sound a lot like the Momentum, with just as wide of a soundstage. Maybe even a tad wider.
But I also noticed two tremendous differences. A: The build quality of the HD 439 is a lot chintzier and cheaper than the luxurious Momentum. I am making a special point of keeping my best buy receipt just in case the headband breaks on the 439s.
B: Sound quality is not as sharp and pristine and clear as the Momentum. It's like the difference between the iPad 4 and the iPad 1. The iPad 1 had a pretty good looking screen, until I picked up an iPad 4 and saw how much sharper and clearer it was.
Same comparison applies to the sound quality. The Sennheiser Momentum is so crystal clear, with no cloudiness in the mids like you sometimes here on the 439. Just listen to money by Pink Floyd. When you get to the hard rock guitar chords in the middle, the Momentum retains its clarity and sharp audio resolution, while the 439 sounds a little rough and jagged.
This is just after one night's observation. I will break in the HD 439's a little bit more and report back.