Output voltages are only important in a system with external amplifier(s). The high output voltage allows you to use minimal gain on the amplifier(s) thereby decreasing the chance of amplifying induced noise in your system.
Generally, the higher the voltage the better but some amplifiers may not be able to handle the higher voltage of some source units.
As far as the DEH-P8MP goes, it's a great looking unit with some key features (especially when used with MP3/WMA files). It has three preouts each producing 6.5 volts out. Check it out & good luck!