Connecting Cable Box to Preamp

Are you sure that your cable box doesn't have analog (rca) outputs as well? If not, probably the best way to go would be to get a DAC that has an optical input.

My box has an analog rca left & right out . It's a Cisco box from Bright House cable .
If yours does not , I would go optical out to a DAC . I think that HDMI would be overkill for 2 channel audio .

Happy Tunes

The quality of the signal over a cable box is usually pretty low, like 96 or 192 kbps. Assuming the cable box has RCA outs, I doubt that getting a separate DAC will provide much better sound than the DAC in the cable box. If the box has RCA outs, I would start with a set of RCA to balanced cables, like the ones available like the Monoprice for $5 a piece.

If the box does not have RCA outs, I would go with a very cheap DAC, given the low quality of the source. It does not have to have balanced out, since you can go RCA to balanced with cables or adapters.

I do not know what quality your cable provider delivers, but I doubt that this is ever going to be a high quality source.