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...