The SRH1840 headphones are a medium-impedance (65-ohm) design, but keep in mind that impedance isn't the only factor that makes them unsuitable for use with portable music players -- in fact, the headphones' relatively low sensitivity or efficiency also plays a role.
The SRH1840 headphones' above-average comfort levels make them a pleasure to use for hours at a time. The tonal balance is very accurate, bass isn't exaggerated, and the treble is perfectly balanced so it's not too bright or dull. Wide-open stereo imaging is one of the SRH1840s' best qualities, and you'll never feel like the sound is locked inside your head.
I listened to Apple Lossless files on my Mac Mini computer with the SRH1840s plugged into a Schiit Lyr headphone amplifier, but they will sound fine with an AV receiver, too.
Comparatively, the Beyerdynamic T 70 headphones boast more clarity than the SRH1840s, with crisper percussion and finer details, but the SRH1840s have warmer, more balanced bass and midrange.
So which is better? That's a matter of taste, but the T 70s' superior midrange and treble clarity may appeal to folks who mostly listen to acoustic music, while the SRH1840s' more impactful bass will satisfy rock, rap, and dance music fans.
I also compared the Shure SRH1840 headphones side by side with the company's own SRH1440 model. The two share the same wide-open sound field, but the SRH1440s expose brighter treble and a less generous bass balance. The SRH1840s also sound "laid-back" next to the SRH1440s, so the SRH1440s may be a better choice if you prefer a more immediate or brighter sound.
If you like bass, get the SRH1840s. If you really like bass, don't get either Shure; check out the Beats Pro by Dr. Dre, which have more bass and more prominent treble sizzle, but also sound more closed-in and "canned" than either Shure model.
The SRH1840s' wide-open stereo imaging and smooth frequency response were much appreciated when I watched a few movies. Again, these aren't the sort of headphones that exaggerate bass or treble to make a more "exciting" sound -- they just faithfully communicate what's on the movie's soundtrack.
Shure's SRH1840 Professional Open Back Headphones are well-rounded performers. Their sturdy construction, high comfort level, and accurate sound should make them easy to live with for a long, long time.