does output quality change when amp bridged?

The amp does see a halving of the impedence when bridged, so if the amp isn't stable into half of the speakers impedence rating, you can have problems with the amp...especially when pushed hard. However, if your amp is stable into a 2 or 3 ohm load, you are likely good to go.

Also...damping factor is reduced.

Dave

When an amp is bridged it will generally deliver more power than when it is driven stereo into two similar impedance loads. For example one 8 ohm load bridged, vs two 4 ohm loads. This is because when bridged the two channels are operating out of phase so that when one channel pulls from the power supply plus voltage the other channel pulls from the minus voltage.

Also, distortion will not double. Distortion is expressed in percent, so the higher output offsets any higher distortion component. Furthermore, insofar as the distortion and noise is the same in both channels (common mode) for example hum, it will be eliminated in the bridged output.