Speakers are getting distortion

Running an external amp is always preferable to running the internal one on the deck.  The 903u is either 4 or 5v outputs so feeding an amp shouldn't be a problem.  Even though Panasonic may rate the deck @ 22wRMS realistic without clipping it should be pulling 11-13 watts. Think of like this what amp fuse does the deck come with a 10A probably? and what is the fuse on the amp you want to run 2 20A fuses i.e. 40A, but all it's concerned with is ampfliying signal, not tuning or playing a cd just amplifying.  At lower levels the difference between the internal amp and an external amp are minimal but as the volume goes up the difference become very apparent.  If your wires are right, and you're distorting odds are it's the internal amp, not bad, just not best.