Integrated Amp upgrade path

Can’t comment on Mac vs Boulder, that will likely come down to personal taste, but as for a streamer I recommend the WXC-50 from Yamaha, especially if you listen to a lot of live music which may not be available in hi res formats.  I use Spotify Premium mostly because they only have a handful of live GD Master Tracks on Tidal. Yamaha has a very good “sound enhancer” mode which adds back a little of that air/sparkle compression algo’s lose, and is perfect for Spotify users.  Just make sure you use it in “player” mode and bypass the very mediocre preamp section. 
I’ve also used the Soundnode 2i but slightly preferred the Yamaha and its sound enhancer as a streamer. The Soundnode looks much better and I also liked its app better. It may also be better for android users since it has AptXHD in the event you can’t stream over WiFi or Ethernet. 

Yamaha is not really a good match for the OP’s excellent system. Consider streamers from Lumin, Innuos, Aurender, Sonore, Sotm, etc. 

Also Simaudio Mind Network Player is very good. 

Yamaha’s “not good” for his system?  Care to qualify that statement?  
It’s a digital source, format and features matter more then price.  But go ahead and waste your money, op mentioned that “data is data” so I’m assuming they aren’t a sucker. 

Perkadin data is hardly data there canthus be very large differences in what you can get from a dac depending on digital source

As per the op the boulder will bring out the highs and sound way more transparent Vanden are polite on the rop end and mac gear is not very transparent so the boulder should make the vandys more exciting