One thing to keep in mind when using the mac, ipad, or a PC for that matter, for volume control is how the volume is being lowered. On most computer operating systems, the volume is lowered digitally by reducing the bit depth, say from 16bits down to 15, 14 etc. This will of course not leave the song bit perfect anymore. Dithering keeps the music sounding pretty much the same at the lower volume/bit depth, but if you are looking for bit perfect, most times lowering the volume from the preamp is still the better option.
Of course if it is convenience you are after, lowing the volume from the OS is the way to go.