Does using multiple speakers degrade quality?
Fine in most cases...

If the lengths of your wires are short and gauge (thickness) of the wires is large then your configuration will have no noticeable impact on the audio aside from the impedance issue -- which will only reduce power/volume if you design correctly and corrupt the audio if you don't.

But those other cases... ;)

You can run into trouble with phasing at long distances. Since the length between amplifier and speaker A is L1 and amplifier and speaker B is L1 + L2 it is impossible for both speakers to be playing exactly the same sound at exactly the same time. This difference in output is called a phase shift which is a mathematical term expressing the difference in angle between two equi-periodic signals represented in a polar plane (in case you care ;).

It takes sound about 3 milliseconds to travel 1 meter in air while the audio signal can travel through 450,000m (450km) of cable in the same timeframe. When your distances get to 100's of meters phase shift can become critically important. It depends on your configuration and how sensitive your application is to distortion.