Digital streaming options for my system are confusing

I can wholeheartedly recommended Bluesound. I have the Vault 2 but if you do not need or want CD ripping ability then the Node 2 or latest 2i should work.
One other point on the Vault is that it takes hard wired internet only where the Nodes take wireless.

I used to run my Vault via spdif into McIntosh c48 pre which has very similar DAC to the mac6700 although possibly a generation earlier.

It sounded stunning is best word for it.
Have since moved on many times on amps, DAC etc but still have the Vault as to better it I know I need spend a LOT more than the $1050 it cost me couple years back.

Go for it!
You will find that the Bluos app you use on your iPhone to control any Bluesound product is a first rate piece of work. It is extremely powerful and both it and the Bluesound operating systems receive constant updates over the internet.
And unlike some updates I have yet to experience one that was a backwards step.
Good luck and enjoy!

If the 6700 has a USB input it is likely an early version that if I remember correctly was limited to 24/96 max so the innuous is probably overkill on that amp.
Could be totally wrong of course but my McIntosh with built in DAC that looks very similar input wise to the 6700 was limited to 24/96.