If I remember correctly, the Delta was a transport. The Alpha and sigma were the DAC units. I had a Delta and Alpha pair way back when. The Alpha was an good DAC back then. The Delta was a mediocre transport. The problem is that neither the Alpha or Sigma have a USB port. You would need to buy a converter to connect it to your computer. I would just buy a streamer or modern DAC.
I use modern DAC with USB input that is connected to a laptop computer. I simply use Fubar2000 to play files. If you decide to get a modern DAC, check to see if it needs USB drivers. A lot of the newer DACs don't need drivers. And driver availability for new versions of your operating system could be a problem.