Almost all external speakers are self powered and amplified as the output of iPads and iPhones isn't enough to drive passive speakers.
Some headphones are battery powered and have their own volume control inline, especially the noise cancelling ones.
A quick Google search for "inline sound amplifier" finds things like TuneBOOST In-line Mini Headphone Amplifier for Headphones or IMC Audio Amplifier which amplifies the output of the iPad. Of course you would want to insure the headphones you use could handle the output. Looks like there are options out there. I haven't tested these myself, but they may help.
edit, this looks interesting boostaroo
ivan