Some of the higher end ZVOX speaker systems are pretty nice. They will accept a digital input from your cable box.
You could also likely output digital from your cable box into a simple, inexpensive DAC and out of the DAC via analog RCAs, into your pre-amp.
You could also likely output digital from your cable box into a simple, inexpensive DAC and out of the DAC via analog RCAs, into your pre-amp.