You can damage the speakers with too little power, here’s how. I don’t pretend to be an expert, just moderately well informed, so read this.
http://www.clubknowledge.com/Car_Audio_FAQ/?t20
It doesn’t say much about the HU amp being damaged but it can’t be good for it, running it past its intended output (heat too). I didn’t say he can’t use the HU to power the speakers, but I believe it is just so tempting to turn the volume higher and higher because the speakers can take it, until you over drive the amps in the HU. Especially running mp3’s like he will, which can be recorded with all different input signals.
As well, the listening enjoyment will be hampered IMO, if you can’t use the speakers to their full potential. Its like, “I just spent $300 for great speakers that I can only listen to at $100 speaker levels”, if you know what I mean. Then buy $100 speakers.
Yes, you do blow speakers if you give them too much power. But this is not what the problem is here. It’s better to have more power available that you don’t use, than not having enough power in the first place.
It’s like a car engine. If you only have 4-cyl, 100HP engine (amp) but you drive the car at full throttle all the time to get where you are going, then engine will not last long. If you have a V-8, 200HP engine (amp) and drive it half throttle all the time then the engine will last a long time. That’s how I see it. You don’t stress the equipment and over work it even if you have more power than you need, when you don’t use it! Now if the transmission (speaker) is only designed for 100 HP and you drive your 200HP engine at full throttle, you’ll blow your transmission (speaker).
If he doesn’t drive his amp at full all the time he will be fine but it’s money wasted IMO to use the speaker to a ¼ of it’s potential.