On board or outboard DAC?

Well, you are going to need a convertor to be able to connect the two together. Though they aren't that expensive, I wonder if there would be any sound degradation?
As far as using the SimAudio DAC to process the signal instead of the UDP, you may notice a slight difference in sound reproduction as each machine will process the sound differently as per their design. Whether this is 'better', only you can determine.
That being said, if both units had matching input and output, then I would say give it a go. But, since you have to convert the signal in order to get it to the DAC, I would be less inclined to bother. 
Others may disagree, though...

gdnrbob,
I appreciate the response. The more I dig into this the more I feel that I may be best off to leave well enough alone. We always look for that one more tweak to make our systems better. I think I'll just stick with what I have.

Falconquest, I also have the Ayre C5XEmp and have tried my Rega DAC and was about 10 seconds into a song and realized I was not going to do that. The I bought a Oppo 105d for use as a BluRay and 5.1 music. I have tried playing Redbook and 2Channel SACD and again, return to the Ayre. I realize the Simaudio is better than the Rega, but my thoughts are that the Ayre is very tough to beat. I would think a very expensive DAC would probably show improvement but then a risk of jitter may come into play as a new issue.

just my opinions