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.
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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.