RCA to Optical Converter

@jkampmeyer I can confirm I have used a solution like @gkelly suggested, going from RCA to optical for a wireless headphone system, and it did work. Quality was within expectations for such a setup.

My challenge is that each of the speaker outputs has a red & white RCA female jack (a total of 4) I cannot find a DAC that has 4 RCA inputs. Also, I don't think I need a preamp, as my receiver has a phono input.

Confusion here are we talking about the pre-out on the back of the receiver? It appears that there are two, 1 for the left and right front Speakers. The other ones are for zone two set of speakers. Use the one for the left and right front.  Ignore the zone two one for now. 
also, another alternative would be to lose the Bose Sound bar and just get a set of speakers. Can guarantee this will have a better sound outcome than what you’re attempting to do. Look on craigslist or Facebook marketplace. 
Another question why are we not just hooking it up straight to the TV?

It is not a pre-out.  It is the + and - for the left and right speaker.  Instead of using a bare wire terminal for each connection, they used RCA jacks.

So you need an ADC instead of a DAC.  An analog to digital converter.  Here's one.