Music Fidelity + Gamut = problems?

I need a professional advice.
Recently I bought a stereo system: acoustics Gamut Phi3 + integrated amplifier Music Fidelity m6i + CD Music Fidelity M6CD. But I’ve got some problems from the very first days.
After several days of listening I noticed that one speaker does not sound properly – there is no bass, the sound is very weak. The second speaker played well, except for some songs with rough percussions. It chatters on songs like Stranger in Moscow of Michael Jackson when I turn up the volume to 15 mins position. It started to chatter and jar when I firstly put this song on volume of 30 mins position (there is sound of rain and then suddenly starts percussions). The CD is of high quality, so I doubt it caused such thing.
I gave the acoustics back to the seller. They changed a dynamic of one speaker, where was the weak sound. However, they assure me that the second speaker with chattering works fine. It really works fine but with slow music, like jazz. But fails to work properly with rough percussions on loud volume.
I doubt that the speaker is in good condition, because the acoustics have a power margin: the amplifier is designed for 200 W, the acoustics for 300 W. Is it possible to kill a speaker with such characteristics when you turn on the volume no more than on a half?
And is it really possible that such chattering on some songs is OK for pricey acoustics when the volume is on 15 mins. position?

I think that the problems could occur because of non-conformity of impedance – the acoustics has 4 Ohm, the amplifier – 8 Ohm.