if the phones are high-z, then they will need a higher voltage across them to make the same volume. I have high-z phones (sennheiser) and I need a good dual 15 or even dual 24 (one of my amps is dual 30) to get enough volts across them to drive them properly.
mostly, you don't need voltage gain for phones, though; you need current gain (drive). but still, 12v (if that's all he has) is a bit absurd if you are talking about high-z phones.