Optical digital input on old boombox

Hi everyone, i have an old (sort of old, 1998) JVC RV-B90 boombox and the sound from it is amazing. Need less to say i dont use tapes, the tuner is pretty naf and i dont have many CD's anymore but what i want to do is utilize the processing and amp sections which sound great.
At the moment i use my laptop with iTunes and connect using the analog 3.5 at the back which works ok but the laptop also has optical and coax spdif out and it would be nice if i could use either of those (preferably the optical) thus keeping everything digital, atleast upto the amp anyway. Obviously the JVC hasnt got a digital input.
So what i would like to know is there a way of connecting the optical or coax spdif (possibly using somesort of interface unit) into the CD circuit in digital form before the digital signal processor or maybe at another point in the circuit so as far as the unit is concerned it is playing a cd and therefore all the sound processing and amps work as before?
I have managed to get a copy of the service manual so maybe all the info i need is there but this is a little beyond me so if anyone has some thoughts on how i can proceed it would be appreciated.
If need be i will post the manual in the resources section as it was too large to attach here.