"May I ask what's the difference betweend sound flat, lifeless and dull?"

I think I understand your question. The Bose cubes, for instance, are dull in the midrange, but not lifeless. I suppose it would have been better to say that the 301s have muddy, slow bass, dull mids and are lifeless in the higher frequencies. They are fairly large, very good looking, poor performing loudspeakers, IMO.

Enjoy the Music. Trust your ears. Laugh at Folks Who Claim to Know it All.