I also replaced the head unit and added a PAC interface box to keep the steering wheel controls, using those with the stock speakers and amp. It's only OK, tbh; it'll play louder than stock but it sounds pretty harsh (lots of EQ applied to tame the hashy high end) and there's a weird quality to it that probably comes in part from the PAC interface.
Better than stock, but I was a little dissapointed given the money spent.