Why buy a DAC when you have a high end pre/pro?

I've done this for quite some time - PC digital SPDIF output to a Lexicon MC-12 digital in, which uses the internal DACs to convert to analog. Very nice and a minimal number of boxes.

I have recently changed a fair amount of gear and now have a 2-channel preamp without built-in DAC, so I will have to get an outboard DAC. But, I would answer your question by saying if you have a high-end pre/pro with quality DACs, I don't know why you wouldn't do just as you suggest.

More of the audiophile things. Sweeter midrange, depth, imaging, less congestion and so on. As always these sonic features may not be worth the price and hassle but only you know whether it's worth it to you or not.

I found that a $200 Cal labs tube DAC did all these things and worth it to me.

Audioengr: I took the time to read a review on your Proceed AVP (it was very complimentary) and one of the highly touted features was the internal DACs so I have to imagine that your external DAC must be really special.
I did start using my CDP as a transport and noticed an immediate difference (for the better) by using my Krell Pre/Pro as the DAC. I guess you start with the Pre/Pro or Receiver and then experiment from there and take note of what your ears are telling you. Budget also becomes an issue for most of us.

You are right. If you had a high-end processor like the Theta Casablanca, there is absolutely no reason to buy a separate DAC.