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 

seems easy enough to try and compare, both your Ayre and Simaudio are outstanding products...

I believe the interfaces are compatible, so all you need is a balanced to single-ended converter on the Ayre and a digital coax cable in order to send data to the Simaudio DAC. That being said, I also own a C-5xeMP and I haven't heard a DAC best it in terms of sound quality. If I ever listen to Ayre's new QX-5 Twenty, I may sing a different tune.

Just from the perspective of flexibility and adaptability I would opt for the outboard DAC.

That way I can...
- use the IC's I believe provide the best sonic performance
- use the best Power cables  I believe is best suited to the component
- change to another DAC as the technology improves

Inboard DACs may be constrained by "design decisions" that are based more on implementation costs than sonic performance - even in some high end gear unfortunately.
- Also, having the DAC share the power supply with a power hungry amp stage  may not be the best solution, sonically or operationally

Regards...

Thanks for your comments Theo. I use an Oppo 103D for video and have tried it for 2-channel music as well. I always go back to the Ayre myself. That's why I thought I would seek responses prior to hunting up a cable. I really enjoy each piece of gear so as I already mentioned, I think I'll just stick with the configuration I have.