I've checked the obvious things like making sure the HU wasn't muted and defeat wasn't on. I put in a CD, cranked the volume all the way up and put my DMM leads on the RCA jacks on the rear of the HU and it reads absolute 0 volts. With the unit having 4 volt preouts, shouldn't I be seeing 4 volts, or very nearly 4 volts AC? The only thing I can think that could be wrong is the preamp output circuit is bad. What else could it be?