I'm running my music from a power Mac, through an Airport Express into a quite old Denon PMA 250SE and a pair of Wharfedale Diamond 9.1s. The Airport Express has made a huge difference to the quality compared to playing the music via my old laptop's headphone jack but I'm sure it can still be better. Am thinking of either getting a Beresford DAC or maybe a new amp, such as the award winning Yamaha A-S500 or Marantz PM6003 (the amp also powers a CD player). Which upgrade will make the most difference? Any tips and ideas would be most welcome. Cheers.