The key word is Headphone Amp! Headphone Amp! Headphone Amp!
When dealing with High quality Headphones, and portable devices such as IPhone/IPOD you need a Headphone Amp to drive the headphones full potential. The built in amp that's in the Iphone/Ipod is not powerful enough to support the higher end headphones. There's no way the Beats are better than the Denon AH-D2000, you just never felt it's full potential.