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Yeah it'll be fairly easy to bridge the channels for the splits, you just wont have fade control unless you use a pair of splitters at the amp.
Did you get the car back?
The speakers are probably distorting not because of the amp or speakers...but more than likely the head-unit itself is sending a distorted signal. No head-unit will be able to output a clean signal all the way to its max volume output, thats just the way it is. Theyve done pretty well to get 60 out of the head-unit without clipping. I'd say 60 is your max volume level on the head-unit.
Yes you will need to re-tune the amp when you bridge it for the fronts.....as they will be receiving double the original (tuned) power they will distort at a lower volume than 60.........even though they'll be louder the sub wont be at its full potential because of the volume on the head-unit. You'll either need to turn down the gain for the splits or turn up the gain for the subwoofer to match.