I have done it before, you will make a LOT of noise! I did it with an old Peavey bandit (112 W) and could be heard on the other side of the village !

The trick is to run the CD outs into the FX loops return which you have to set to line level.

You also need to sum the L and R outputs from the CD player. Best way to do this is with a desk or something that can accept a stereo in and will reduce to to a mono out.

Schmange it totally right about the speakers / cabs, if it starts sounding distorted, ease back on the volume !