Is My McIntosh C52 Pre-amp The Weak Link?

If you have the DA1 - I agree with @fastfreight the you have found the weaklink.

I have owned the C2600 and when I add Moon 280D as a streaming DAC it was a nice step up.

Time to step up to a separate DAC and stop with the onboard DACs in preamps and streamers.  If you detail what specific improvements you’d like to achieve you’ll get some good recommendations here. 

I agree, if it’s the DA1 you can do much better.  I have the DA1 in my MA8900.  I now use a Pontus II that is much better.  With the quality of your other components I would look at spending $5k or more on an external DAC that matches the level of the rest of your system.  See if you can find a local retailer that will let you demo different DACs in your setup then you’ll know for sure what, if anything, you are missing.

@soix  is correct.

@jastralfu makes a very good point

I would suggest having a stand-alone DAC also. You're just swaping out your issue (The C52 having a built in DAC) with a new issue (having the DAC built into the Streamer). 

I am not a big fan of seperates either (I own an integrated amp), but have found disappointment everytime I had a DAC built into another component. I own a N20 by the way, with a PS audio DAC (version 1).

I would also suggest listening to several DAC's, I have found they all sound different.

IMHO, keep the Mac, but upgrade the DAC.