I got my Harman Kardon 4550 set up on Friday night, and tried a few movie soundtracks on it as well as a CD or two. Sounded great. However, I've now got my PowerBook plugged into it via the Video Aux 4 input on the front, and I'm using iTunes to listen to music until I get my new iPod to play MP3s. That's where the problem started. The music seems to lack weight, with mid-tones becoming weak and the trebles going to the harsh side of bright. At first, at low levels I didn't notice. But when I turned the amp up a bit, the music just didn't involve me at all. So I went to my PowerBook and plugged in headphones instead - it sounded similar, but...well...better. I didn't know what to make of this. So I tried a few movies out, and they all sounded fine. Maybe not as impressive as I was expecting, but fine all the same. My one CD (!) sounded OK too, although I had, by this point, become paranoid that the amp was broken, so I began to analyse every tiny imperfection in the recording. Was the brightness of that guitar the recording's fault or the amp's? The one thing that occured to me was, that the PowerBook must amplify the sound internally before pumping it out of the headphone socket. So, by plugging it into the amp, I'm essentially amplifying a pre-amplifed source. Could this effect the sound adversely, and account for the degredation I've been hearing? How do headphones differ from speakers in that regard? Finally, how likely is it that I've damaged the amp? I haven't played it excessively loud for any extended period. It's all set up correctly and wired correctly. On the other hand, my speakers could have blown, but I wouldn't know how to diagnose this. Basically, what are the tell-tale signs that something, be it an amp, or some speakers, has been damaged? Sorry for the unfocussed rant, but it's stressing me out!