Computer to adcom amp/preamp

After the tot broke the CD player, the stereo system went into mothballs. Fast forward 15 years and he is recording music into a Mac mini using a tone port connected by usb. Hoping to use our old Adcoms and Bose acoustimass 5 speakers for playback but the only output from the computer is an aux cord for little headphones or the tone port's big headphone electric guitar-style jack (sorry I don't know the technical names). The adcom preamp GFA535L has left/right inputs. Do I just get some kind of connecter that splits the output signal? Will that sound ok? Any other suggestions?
Run a cable from the headphone port on the Mac to one of the inputs on your preamp.
To elaborate on the above--you need a cable that has a 3.5mm stereo plug on one end which will go into the Mac. This cable will break out into two (2) RCA male plugs which will plug in to the Adcom. Can be found cheap and in various lengths at Parts Express.
The GFA535L is an amp, not a preamp...the "L" stands for "level" meaning level controls on the front of the amp, and you could run directly but a preamp would be a better idea.
Two additional suggestions:

One could buy a Schiit Audio SYS passive pre for $50. Would allow for an additional input to the amp.

One could buy an inexpensive USB DAC to run into the passive pre or directly into the amp.