Congrats on the 2100, I think you will be happy.
Cables- You are correct, it is a big debate. I believe that even among strong cable believers, it is agreed, that the difference between cables is minor in comparison to things like speakers, amps, dacs. I have heard minor differences between cables, but never anything significant (unless you count a connection so corroded it barely worked). At where you are, I would keep your cable budget low an focused on quality.
Blue Jeans cable makes excellent cables for reasonable prices. I also recently became aware that Zu audio sells overstock and NOS cables on ebay and you can often get a nice pair of speaker cables for less than $100.
External DAC- It depends. If you want to try, order a Schiit Modi for $100 (or Modi Multibit for $250), and see what you think If you don't like it, return it within 15 days and pay the $15 restocking fee.
Subwoofer- It can be a mixed bag, mostly as I recall the Pinnicle subs not being the cleanest. Worst case, try and take it out of the system. Consider a good, inexpensive newer sub like the RSL Speedwoofer $400, or Rythmik L12 for $540. Use the crossover in the Parasound, starting at a high pass of 80hz for your mains and low pass of 80hz for the subs.
SMGa- Owned a set for years powered by a Hafler DH-220 before moving on to larger Maggies. A bit closed in the treble, but an amazing midrange, perhaps better than even current Maggies in that regard. I think you will find they sound far better powered by the Nakamichi than the McIntosh, as the Maggies do like a good amount of power.
Finally, setup, setup, setup. Time spend dialing in the placement of the speakers and subwoofer, is the biggest difference in sound quality. A $200,000 system with speakers just thrown in the corners will sound worse than a $1000 system with placements dialed in. Read about the sub crawl for placing your subwoofer.