As far as headphone amps go, they make much more of a difference for headphones that are less efficient and/or present a tougher load. The Sennheiser 580 has an impedance of 300ohms, nearly 10x your typical 32ohm headphone, so it has much greater resistance to current and can lead to a rather anemic listening experience from your average headphone jack. Try listening to them out of a discman and you'll notice that first you have to turn the volume way up to get an equivalent sound level as with a 32 ohm headphone, and second you'll probably notice that they sound rather flat and dull. Hook them up to an amp with a good headphone section or a dedicated headphone amp designed to be able to handle the inefficient load, and the sound fills in quite a lot.
