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.
Get rid of the National. Although stamped 7308, it's really a 6922 (a poor one at that). Yamada will build the 5000 on a "special order" basis for use with a E188CC (7308) but the standard is a 6922 because of availability.

I'd recommend a NOS Amperex Holland, Tele or Siemens&Halske (1960's) depending on the rest of your system and personal preferences.

You have one of the finest DAC's on the planet and will enjoy it for years to come. The transport and power cords also make a big difference.
Also, the digital cable is key. Although I haven't tried them all the Purist Dominus is recommended, esp in BNC mode, it is far larger more transparent a stage than the RCA. I would absolutely suggest you use BNC to the DAC from your transport. rcupka is right about the tubes and also the PC's. You can get big gains, just make sure they're matched triode sections in the 6922. I use a Siemens myself and vintage RCA on the rectification. Feel free to email me or post if you have any other specific questions. Also, don't try to skimp on your transport. You can get great results with a fair one, but not be hearing all your DAC can do.
I just picked up a used 5000 Mk IV to replace my Kora Hermes II, which I ran with Siemens CCa's. I love the Siemens' tubes and was wondering if I could swap one for the National 7308?

I'm a little worried about messing around under the hood of the Zanden, given the price I paid for it and my relative lack of experience with tube rolling. Assuming the CCa would be a good replacement for the National, would the swap be a very difficult to perform with the Zanden?