Please help: best streaming solution for McIntosh Stack.

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

I've had really great luck with my Roon Nucleus Plus server, which feeds an external DAC, then into my McIntosh preamp.

I see the Roon Nucleus is $1459 and you could snag a decent DAC, like a Mytek Liberty DAC for $995 and that would give you quite a nice streaming setup for a reasonable price.  The Nucleus would run the server side.  You would install the Roon app on your smart phone or tablet (or both), to play music from your source(s). I sometimes use the app on my Android phone, sometimes use the app on my iPad.

The Roon server has two USB outlets, which are dead quiet.  I use one of them to connect to my DAC and the other to connect an external drive (which stores the music I've purchased).  For just streaming, you won't need the hard drive.

All you would need to add are interconnects.  The Roon server comes with a year's subscription.  After that, I believe the annual subscription to Roon is around $120.