Integrated Amp upgrade path

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.  

I have a Moon by Simaudio 280D with MiND2 a significant upgrade from my Node 2i and the DAC in my C2600 preamp. 
Lots of options of course depending on your budget. And the Boulder DAC/preamp could be an option too. 

@arafiq my experience with the Node 2i was positive.  It's $600 with great features.  It was my dipping of the toe in digital.

When I upgraded my preamp - McIntosh C2600 - part of my reasoning is the internal DAC and phono stage (MC and MM).

I set up the Node to provide go through the DAC as well as provide an analog signal.  I would switch and do an A and B comparison and at times there was a difference.  Enough to make me wonder if there was value in upgrading my source given I upgraded the preamp/amp and speakers.   

So I decided to evaluate the impact of upgrading the DAC and compared the Moon 280D to the McIntosh internal DAC (version 1 non-modular) and the Node 2i

I was surprised by the improved details with very dense music such as Black Sabbath War Pigs and on songs with delicacy like Steely Dan's Hey Ninenteen or Chuck Mangione's Feels So Good on the Moon 280D.  It was significant over both the McIntosh DAC and the Node 2i DAC - so I decided to go for it.

I found the MiND2 easier to set up than the Node 2i (specifically with my NAS).  I've been impressed with Moon customer services support - they respond to questions quickly. 

Based on my experience, Moon is an excellent value for the money.  If I ever update my DAC/preamp/amp...Moon separates or integrated will be on my auditioning list. (in addition to Audio Research and McIntosh)

I like my McIntosh and I was able to buy it at a discount because they were demo models - I've heard Moon products with Davore Super Nines and Wilson Sabrina X speakers and I have been impressed.  

@arafiq No I have not tried the Mind2 as a streamer into the Mac DAC.  The MiND2 is integrated into the the Moon 280D and its output is analog only (balanced or RCA)