I'll post comparisons to other headphones later as I get a chance to listen more. Haven't had a lot of time to myself this week.
To preface the comparison, I'll just say that I love the recabled-HD650s and IME, nothing comes close to them for the price. I still think they compete in several areas with the best headphones available.
Anyways, the 1840s have been burned in for about 50 hours now, but I honestly stopped noticing changes after the first 15-20 hours.
Stock Shure SRH1840 vs. Sennheiser HD650 + DHC Molecule UPOCC copper cable
IMO, the strength of the 1840s is in their tonal balance. Natural/realistic, balanced, and forgiving. They really just sound good with everything. There's no peakiness in the upper registers and nothing sounds out of place. This is a rare feat that I don't think many headphones have accomplished, regardless of price. The HD800 and T1 certainly haven't for example. They required recables and gear matching to be palatable; things which I don't think are necessary for the Shures. That's not to say they wouldn't benefit from an aftermarket cord, but they don't seem to need one to sound "right".
Both headphones have a natural tone, but whereas the HD650 can sound a bit dark, the 1840s remind me of an HD650 with better treble quality and balance, less bass, slightly more prominent mids and a bit more airy sound. The Shures also have one of the most mid-centric sounds I've heard, but their focus is higher in the midrange than on the HD650. I chose to use the HD650 here because, of the cans I own, they are most similar to the 1840s in tone.
Little more on the bass; the HD650 has more punch and moves more air. It can deliver more force and sustain it better. Speed is similar, but I'd give the edge to the 1840. The decay on the HD650 is a bit too drawn out and sounds a little smeared/fuzzy in comparison (subtle effect).
By the way, the bottom end extends all the way down on the 1840s (tested to 25Hz). They sound quite linear throughout the bass and it doesn't roll off as steeply as on the HD650. This is very noticeable with sin waves. However, the flipside to this is the fact that the HD650 has a more rhythmic and involving bass because of the properly implemented midbass hump. Caveat here being that the 650 can overdo it at times and become overwhelming with bass quantity, a problem not present on the Shures.
The 1840s deliver a little more detail in the upper mids/treble but their soundstage is flatter-sounding than the HD650s, which sound a bit more holographic (less "wall of sound" effect). The HD650s also have more pinpoint focus and imaging but the difference is subtle and took a while to nail down. The Shures have a wider soundstage, while the Senns offer a bit more depth in the forward plane. The 1840s also have a "larger" sound than the HD650, as if one is viewing a larger sized image in front of them. As for transparency, I'm having trouble comparing the two; both headphones are almost equally adept at conveying variations among recordings and equipment. If pressed, I'd give the edge to the 1840s here.
Where the Shures win unequivocally is the smooth and seamless integration/transition of the mids and treble. Compared to the 650s, there's a lack of graininess and scratchiness and the mids/treble are more liquid. All of these factors often contribute to and translate into a more involving and less fatiguing experience.
Comfort-wise, the Shures are even lighter than the HD650s, have much less clamping force, and softer earpads. Actually, I think the earpads might be a bit too soft. Yes they're comfy, but I wonder about their durability. They deform pretty easily and some parts hug the ears more than surrounding them like the HD650 do. I don't think it's a problem, it's just different. Fortunately, there's also an extra set of pads in the box. The build quality on the HD650 seems somewhat better because of its higher-quality and density plastic and other materials. The tradeoff ofcourse is weight. The added use of light-plastics on the Shures make them super light and comfortable. I'm not worried about the 1840s durability though. They're not at all flimsy/creaky but just don't feel as solid and "high-end" as some of the other heavier headphones like the HD800, T1 and even the HD650 as described earlier.