That being said, am I leaving a lot on the table?
Yes. See below.
Would a dedicated streamer sound a lot better than the Macbook Pro?
Yes. Most definitely.
Are there streamers without DACs, so I can use the McIntosh DAC without purchasing redundant equipment?
Yes. Plenty.
Can I get a streamer that uses the DAC of the c2700 preamp?
Yes. You will connect the streamer via a digital cable to the built in DAC in your preamp. Such as USB, S/PDIF, or AES / EBU.
Finally, do I then need to add another subscription, such as Tidal or Qobuz, to get high res audio?
Not necessarily. Apple Music (red book standard 44.1/16) is pretty good already (CD quality). You can take a free trial for Qobus or Tidal to determine whether they are any better for you and your tastes.