Looks like they figured out that the Beats setting was overcooking the amp (by boosting volume too much) with some phones and toned it down.
All Beats Audio does is jack up the volume, slap on a bass and treble boost in effect. Before the changes, they were really jacking the volume up, presumably to show the 'sound quality difference' (ha) - and that was making it a struggle for the inbuilt amp to power certain headphones in Beats Audio mode.
They've kept the prominent bass & slight treble boost, and they're not making it as loud in Beats mode. A better compromise IMO for headphone users. Earphone users are best off sticking to lower volumes. Even with the less overcooked amp, with a 32-ohm headphone of regular efficiency, >18 in Beats Audio mode is already at longer-term-hearing-damage levels.
With the bundled 8x earphones, as the above poster says, you'll be hard pressed to notice a major difference, the phones are pretty regular-issue pack-in earphones in terms of general quality and bass capability (and seem to be not very efficient, so actually safer to use from a hearing damage perspective).