Digital Advice

Check out the BlueSound Node N130 or newer. It streams Amazon and does great things for little money. New or used.

All the best.

Use your Mac to stream to Schiit via USB. This will beat WiiM and Bluesound. 

I'm not sure. I had been streaming from my Mac to a DAC for a long time before switching to dedicated streamers. I would suggest streaming from a Wiim to a good DAC instead. The weak link in both the Wiim Pro+ and Ultra is their internal DAC, but the RC and PEQ (with a good mic and proper setup) compensate for it, making them almost indispensable yet very affordable hi-fi gear.

The Blusound node can be bought for a couple hundred on the used market. The app is the best. I don't think there is a better low cost solution. I don't know if $600 elevates you much. 

Without going overboard - audphile1 has a solid suggestion. You might consider a new Schiit Miodi instead of the used Bifrost. It has a nice USB implementation that will isolate the DAC frrom your Mac.

https://www.schiit.com/products/modi-plus

With the budget you have, what I would do is drop the $600 on a used DAC and serviceable USB cable for the repurposed MAC.  Right now, that would be the biggest bang for your buck and give you a better upgrade path for the future.  And if you don't want to rip all your cd's to the MAC, and assuming the Denon 2900's problems are transport/laser based, then find a cheap, used CD player with coax/spdif output and connect it to the new DAC.  Then save up some money to add a dedicated streamer later.

• The Schiit Bifrost is a good, solid option for the DAC.  Make sure you stay with the Bifrost 2 or Bifrost 64/2.  There are other DAC's within your budget as well.  Just do your research first.
• +1 mahler123....Stay away from the older Bluesound Vaults.  The DAC's are old and they already have too many years on them.
• The newer Bluesound Node's are solid and you can pick up a used N130 for about $300 (or even a brand new Node Nano at $300).  Copy your CD rips to a USB drive and plug that into the Bluesound. You can then feed that into the better "new to you" DAC.

my 2 cents...