Mac Pro audio for FCPX
Since the speakers are un-powered you will certainly need either an amplifier or a receiver that can handle the Toslink.  Honestly, if you are trying to keep the cost as low as possible, you may have more luck buying new speakers that take digital input.  

Toslink input on a receiver is a mid-range feature if not a high end one, so you probably aren't going to find a lot of cheap amps with it and depending on how good the speakers are, it might not cost that much to replace them with a purpose built system. (For example, Behringer makes this set.)

Alternately, you could go with a simple analog amp.  Toslink itself is more consumer targeted and probably isn't going to give you that huge of an advantage over an analog output unless the DAC in the MacPro is really poor, though even if it is, when you are working on a budget, you probably aren't going to get a substantially better DAC, so investing instead in a basic clean analog amp might get better overall results.