Integrated Amp upgrade path

Do you mean a MA8900?  I never heard of a MC8900.  I would expect the Boulder to have a much different musical representation. What is it you don’t like about the McIntosh?

I have heard several McIntosh Integrated amps and personally have Mc preamp/amp set up.

I would focus your upgrade path on the source.  I'd evaluate purchasing an external streaming DAC.  My logic is based on cost and optionality.

Upgrading the the Boulder is a significant cost.  You will hear a difference in sound because it will be a completely different source to amplifier to speaker path.  

The DAC on your MA 8900 is good - not great.  In my opinion you can get a significant upgrade for a fraction of the cost of the 866.  Heck the 816 is an option too!

Good luck and enjoy the process.  

Yes, MC was a typo.  Really nothing about the Mac I don’t like—just have been very interested in Boulder.

Had not given much thought to upgrading the front end—-kind of in the data is data camp, but admittedly without much knowledge.  Will give that approach a good look.  Any streaming DAC recommendations?

Thanks to both responders!

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.