It really depends on the amplifier being used as well as if there is any noise present in the install. But it's not exactly a huge pricejump to 4V preouts, they're pretty much a standard now. These 8V and 16V HU's being released these days by Eclipse are pretty gimmicky IMO. Not only are few amps designed with this high a voltage in mind, but with many amps you will have to go out of your way NOT to clip it with a 16V signal. But I'm just rambling now so I'll shut my smelly mouth...