Zanden 5000mkIII dac tube recomendations

I have a Zanden 5000mkIII dac in stock factory form including Zanden supplied tubes. These are, in power supply, Raytheon JRP 6X4W and 2 Sylvania 6CA4's. In output a Natonal 7308/E188CC(USA). I am not really looking to change the character of the sound, but wonder if I could get an overall "better" sound out of this very special dac. Part of my inquiry is instigated by my limited tube knowledge suggesting that the tubes in at the moment are a little run of the mill-or have I missed a key factor in their particular selection by Zanden? I would really like to sort this out and any comments or particular experiences would be much appreciated.Thanks.