When looking for a head unit, how important is preout voltage when hooking up to amps? What else is there to look for in purchasing HU's with an amp set up in mind?
Look for the features you want/need first. Do you need dvd? MP3? Surround sound? Most HU's will give 4v preouts and that's fine with most amps. However, some will give 6v - 8v. Those cost a lot more. Higher volts will give cleaner sound.
Like Isaac said it makes a cleaner signal to the amp if the voltage is higher. Some of the low - mid grade H.U.s have 2v preamp outputs as well.
One more thing, Make sure your amp can handle whatever voltage you are putting in it. For example, A certain MTX amp can only take up to 5 volts of input power. So an H.U. with a 6 Volt rating is too much. Im not sure if there is a way to lower the voltage or not on an HU. Maybe sum1 else knos