There's nothing wrong with the Iphone 4's bass, it's flat, reference quality it doesn't roll-off like other players. Perhaps you just like the coloration of the Fiio. It's not about current with customs, most are very efficient and sensitive, it's about trying to get the whole system controlled best as possible which low output impedance players will be able to do. The Iphone 4, Clip+ and S:Flo2 have the lowest output impedance (OI). If a source has a high OI, it will bring about dips and humps in the frequency response, the IP4 is among those players that don't do much of this because of it's low OI.