It depends on the amp. Some amps like a home receiver may already have some bass boost options turned on or be bassy to begin with. Other amps have high output impedance, the resistance at the headphone jack, that can cause poor control of the headphones especially near the bass fundamental frequency which tends to lead to boomy and distorting bass.
In other cases a headphone may have low sensitivity ( given in dB ) as well as impedance issues making some amps not capable of properly driving the headphone and voice coils. In this case your headphones may seemed tame when in actuality their sound signature may be boomy and your portable device for example is not properly driving the headphone.
This is why it is so important to list your gear when you ask a question or make a review as gear can alter the sound signature.
To further answer your question we need to know what equipment you are using, what settings you have, and if you are utilizing any EQ.
Edited by NA Blur - 5/10/13 at 10:31am