Is My McIntosh C52 Pre-amp The Weak Link?

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.

I was revisiting this today and thought that you could always purchase used from somewhere like The Music Room or Upscale Audio.  They have a great return policy so if you get something that doesn’t work well in your setup you can return it.  There’s no shortage of top level DACs you can choose from.  I’ve only had the DA1 and the Pontus II with a Hermes DDC so I can’t tell you what anything else sounds like.  If you let folks know what characteristics you’re looking for you’ll get plenty of recommendations.  Some of the names I see around here are Denafrips, Chord, PS Audio, Weiss, Mola-Mola, Lampizator.  There are many others I’m forgetting.  There are also R2R and DeltaSigma DACs and one may suit you more than the other.

Separate DAC gives you much more flexibility.