Digital Advice

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...

 DAC & Mac = 5

 Streamer DAC = 8 votes

After reading the many great advice my thought is I need to decide if this is an end game purchase or a start of a journey. An end game purchase requires 2-3x the budget I have now. Even those who suggested a streamer DAC at the $600 budget hinted at adding a separate DAC as an upgrade path.

One thing I learned here in selecting a streamer is to pick one with a good app. I admit I have to be educated here. I have only used the Amazon app to stream music & Apple Music to play burned CDs.