Please help: best streaming solution for McIntosh Stack.

If you want to be able to store files on the music server, take a look at the Aurender N100C (with Coax and USB outs)  for around that price. I had a really good experience with it sound and workability wise.  It can also interface with NAS and external USB drives.  Its conductor app works well with Tidal and Qobuz.  Aurender has other models like the N100N (with just a USB out). 

Innuous Zen is another alternative in that price range.

If you don't care about saving files on the unit, you can consider the Auralic G1 or the Lumin U1 Mini streamers.

Hope that gets you started. Good luck

No need to save "on the unit" if it has USB in.
Unless you enjoy paying up.
The"sound" of the streamer is determined by the DAC. You have to find one you like.
With Mc I fear that looks enter into the equation.  Why not an MS500?
Hits your $6K mark.

I bought a Bluesound Node 2, replaced it with the Node 2i and have never looked back.
- plays digital sample rates up to/including 24/192
- plays most streaming services
- performance improves with great IC's and power cables
- nice, intuitive interface that’s available on PC’s Tablets and phones
- "The Vault" version has a hard disk for your CD music
- plays from Ethernet or Wifi
- plays music from streaming sites, blue-tooth devices, USB devices and NAS drives
- regular software updates install once you confirm and work flawlessly

It is very affordable, sounds great and easy to install and use.
You can always add a DAC later if desired, but mine sounds amazing without one.

If you want to use it in wifi mode then I would recommend getting a good wifi router and not the one often supplied with your internet provider.
- I did just than and I have great wifi all around the house - even in the back yard - I also have the Pulse mini speaker for outside music.

One little caveat - some people have reported a hum when using the NODE2/2i - this is due to it having an isolated (2 pin plug) power supply - and mixing ANY source device of an isolated design with some amps can cause hum issues.
This can be remedied simply by grounding one of the inputs on your amp/pre-amp to a ground point on the house outlet
If you do not get a hum then your system is properly grounded

The Node 2/2i has a small footprint, so If you feel you would like to step up to something more "substantial" take a look at a sister product from NAD - it uses the same BlueOS interface

Regards - Steve