Volume on External Speakers via Headphone Port very low

Hi,

I've just received a set of Harmon Kardon Soundsticks III for Christmas.

Great Looking speakers. I just need them to work ;-)

I have them connected to the headphone out port on my Mac Mini.

The voume out is at max and I'd describe it as very low. I can hear it clearly, but for playing music as intended it is too quiet.

I've gone into the Midi Setup application and tested the speakers. They blast out a much louder noise when I test them.

My Mac Mini is connected to a 27inch Thunderbolt Display. It has built in speakers and they are considerably louder. (But tinny in my opinion.. hence the external speakers.)

I've tried resetting the PRAM (that's also a problem as the mini hangs on a grey screen when it reboots resetting PRAM... don't know if the problem is connected...)

Anyway, has anyone experienced this before and do they have any advice? It would seem that the operating system is limiting the volume out on the headphone port.

Any help appreciated!