Matthias, a £2.5k dac will surely make the financial pill of Extreme easier to swallow. Life being what it is, there is the possibility or likely probability that Extreme could shred the Yggy. If it doesnt, and Yggy a good holistic match, does that say more about how great Extreme is...or Yggy is?
I have built music servers for friends with DACs that have varied across the board including the Yggy. Without exception, a good music server has improved them all. I have also heard very good DACs that have failed to separate themselves more than one would expect from lesser DACs because of a poor server.
What I have found in my own digital journey is that a poorly-implemented server can sound harsh, bright, and unnatural sounding even when paired with a very good DAC and while I do believe that the DAC has the potential to make a greater difference than the server (since a DAC is both a digital and analog device), as we rapidly approach 2020, even a $100 AQ DragonFly DAC has the potential to sound quite engaging when paired with a good server and so the lowest standard of DAC that can be purchased today is of a higher standard than the lowest standard of music server.
If I were to advise someone starting from scratch with a digital system, I would say get the best music server you can afford at the compromise of the DAC and then later improve the DAC as finances permit. What you may find is that even with your inexpensive DAC, you may find yourself so pleased that you may not wish to bother or that you don't necessarily need to climb to the highest echelons of DACs to achieve the level of SQ you were hoping for.
The highest echelon that one can climb with respect to music servers at this time, imho, is the Extreme at about $25k USD. With DACs, you're talking well north of $100k and so it's just more prudent to start with the Extreme even if your goal is to reach the very top of the mountain with respect to a DAC. My audiophile group meets periodically to listen to new pieces of equipment that someone might buy and this inevitably leads to shootouts. What is interesting is that there is no consensus in our group on what the best DAC is as it turns out this is quite personal whereas when we hear a good server, we are more likely to be unanimous in our preference for that server. This is no more evident than on this thread where you have people who own DACs across the board and yet they have all come to the same conclusion as to which music server they believe is best.
While it might seem ill-advised to mate a $25k server with a $2.5k DAC, I have absolutely no doubt the Extreme will make that DAC sound like a much more expensive DAC. I don't think this can be said if things were reversed. I don't believe a Select II, for example, will make a NUC sound the equivalent of the Extreme.