You didn't say which outputs you were using, home or car? The car outputs (well, the sled outputs specifically) are definitely 4v.

As far as the home outputs are concerned, I drive those all the time at exactly 0db and never have any trouble with most amplification equipment. Occasionally I'll run into a receiver that clips a tad and then I have to bring the player's volume down a couple of DB, but otherwise it's not a problem.

Check to make sure your EQ is flat or below 0 on all bands, and that you haven't used the Hijack bass/treble commands to increase, only to decrease. If you increase any of those settings above 0db, you'll get clipping at the DAC on very loud songs.