A21 vs McCormack?

Everyone -- thanks for your responses; it's a big help in deciding how to move forward.
@labtec: thanks for your detailed message and suggestions.  I'm having some trouble with my DNA-1 now, so I don't want to put this off any more; the issue was whether to send it to SMc, or go with another amp; given all the current options with Class D and other lower cost solutions, I thought I should explore it a little before going with the rebuild.  From all the responses, I think the rebuild is the right way to go.  However, I appreciate your advice on setting my other priorities; I have been fairly happy with the Benchmark 1, but I realize it's not SOTA.  How much do you think I should budget for an SOTA DAC that would complement the rest of my system, and would you recommend new or used?  As for the subwoofer suggestion, that is on my radar, although I'm planning to go for a swarm after I get the amp and DAC budgeted.  Thanks again!

@cheeg, 
For the money, I would get an Ayre Codex for a DAC.
Used, they are just a bit above $1K these days.
Or, a Schiit Gungnir or Yggy.
Both companies offer upgrades, and have very detailed sound, in my humble opinion.
Also, if you can ever score a broken DNA-1, SMcAudio can convert them to monoblocks. All they need is the case, everything gets scraped and overhauled. They did a pair for me a couple of years ago.
Just be sure to tell Pat or Steve what equipment you are using, they can tailor the amp to whatever you own.

As far as Class D goes, I would give it a couple more years to mature.
Yes, they can sound great, but I think staying with A or AB amps would be the safest course, for now. Again, just my opinion.

Bob

Just a side bar, but the A21 is not made in China.  Joecollege is plain wrong and all he had to do was a simple fact check before typing.