Which DAC - Streamer or Preamp?

I thought about jumping to the A8 right away, but its an upwardly slippery slope

If it was me I wouldn’t do that — I’d get a separate DAC to use with the A6. DACs are a personal choice and sound different, and what are the odds that the DAC in the A8 is best for your particular sonic preferences? I’d say very low. There are tons of excellent DACs out there today so worth doing some research to find the one that best matches your sonic preferences and do that. I don’t believe there’s much difference in streaming between the A8 and A6, and the A6 should be fine as a streamer for now. Or maybe you’ll get lucky and be very happy with what you already have.  Just my $0.02 FWIW.

@soix that’s a great point, and one that didn’t occur to me right away, coming from a more common audio consumer mindset, thanks.

99.9999% you have DA1 DAC module in that McIntosh unless someone upgraded it. That DAC is nothing special at all. It’s easily bested by a good outboard DAC such as Benchmark DAC3 that you can buy used for under $1200. Compare the Eversolo DAC to DA1 and determine for yourself. 

the DAC1 card is just ok but the DAC2 baord is very good in comparison. probably need a $2500+ outboard DAC to beat it. You may want to price the DAC2 board upgrade for your mac, it also adds some very nice conectivity for a TV through the e-arc input also has usb input, very flexaable. 

Not to bag on your older Heresy but I did have Heresy IIIs. Not sure that's a system to tell minor differences. So yes, try all the ways you can hook it up and see what you think.