I'll skip the accurate but not helpful 'like listening through a sheet of sludge' sentiment, but what Beats does is a loudness + EQ preset. And what could be happening with a Beats-on-Beats headphone setup is that if you're listening very loud, you're encountering either clipping or other similar undesirable effects of having it cranked to the max.
The 'bug' mentioned above may be that the Beats setting currently cranks it up too hot - although my lower end reference MDR-7520's (which have a somewhat Beats-like sound signature) don't clip at normal listening volumes with Beats enabled, there's an awful lot of resulting bass boost going on for that tiny headphone amp inside to deal with when faced with driving a respectably-sized headphone transducer.
The reason may not happen with earbuds (notwithstanding the 'listening through shiny sparkly sludge' effect) is that they're a lot more efficient.