There are a many possible causes of the symptoms you have described that are more likely to be responsible for the issue than the fact that the speakers have a 6 ohm nominal impedance instead of an 8 ohm nominal impedance. One such factor being how the impedance of the particular speaker varies from its nominal value over the frequency range. Another possible factor being the output impedance of the particular amp and how it interacts with the variation of speaker impedance over the frequency range. Another possible factor being an impedance incompatibility between the preamp and the amp.
More power may not be the answer either.
What I suggest is that you indicate the specific make and model of the speakers, the amp, and the preamp or other component that is driving the amp. Some idea of the kind of music you listen to, how loud you listen, room size, and listening distance may also be helpful. Hopefully that information will enable us to provide suggestions that focus on your specific situation.
Regards,
-- Al
