How long it take to run in brand new loudspeaker?

I think that it is usually the other way around. Speakers that haven't been broken in seem to be too bright / harsh high frequency characteristics. As they break in, the high frequencies should mellow out a bit. So, not sure about your situation. It may be that these speakers have the general characteristic of being soft in their high frequency response??????

my experience was just the opposite of Bufus - new out of the crate my speakers (not quads)were very rolled off - muffled in fact. i didn't count the hours but i let them run for 3 to 4 days and the opened up just fine. be patient.

It really depends on the speaker and how hard they're run in. 50 hours at a decent volume will make most speakers start to sound mature. Many speakers take a good 100-200 hours to really sound their best.

My experience was the same as Bufus and I have found that a few hundred hours are needed before reaching steady-state sound no matter what low number the manufacturer gives for break-in. You may just have bought what are laid back speakers to your ears. Arthur

Aren't there two issues here? Maybe some tweeters have flexible surrounds, and thus will increase in output (get brighter) with breakin; likewise, and more commonly, woofers and mids surrounds and spiders nead to flex a LOT to break in, thereby FILLING IN the bottom. Maybe some do BOTH, widening the "shoulders" spectrally, as it were?