Connecting Cable Box to Preamp

I'm setting up a Krell KSA-250 amp with a KAV-250p preamp that has balanced inputs. My source is classical music from my cabel box. The box has optical and HDMI imbedded outputs.

What do I need between my cable box and my 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
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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.
Do the satellite companies have better signals? What about a sate;lite radion receiver?
No expert, but I think the satellite signals are worse. The cable music is fine for background music. Do you have RCA outputs? If so, spend $15 and get the Monoprice RCA to XLR cables and give it a listen. A little more detail on your box and provider would help.