The speakers may not clip, but the amp may well...

Say the HU outputs a 4V lowlevel signal. Say his amp has a gain range of 0.25 to 8V
Setting the gain to 0.25V (max) would mean that the HU could only reach 1/16 of max if he's playing a full amplitude test tone (normal music is rarely at full amplitude, though normally at some point in a song it is (normalization)) before overdriving the amp.

/Michael