Audible differences in what some call speaker control, and clarity are predominately about frequency response, IOW loudness. The most valuable upgrade that a headphone amp generally provides is firstly low source impedance. The ability to produce > 1 vrms into a headphone load runs a close second. This is because most headphone jacks on desktops and laptops provide a source impedance of 50-75 ohms which is inadequate for headphones whose impedance bottoms out at less than a tenth or fifth of that. Other than specialized hardware, most portable digital players, cell phones, smart phones, and tablets as well as laptops and desktops can't produce more than 1 volt into any load without outboard assistance. Many are inherently limited to as little as half that.