Hi Rok - to answer your questions about reverb in concert halls is not necessarily easy. What I assume they mean by those particular numbers is how long a chord, say the last chord of a big piece, will ring in the hall AFTER the musicians have actually stopped playing it. Reverb is a very necessary element of a good hall - too little and it will sound dead, too much is also undesirable. There are way too many architectural/acoustical factors that go into it for me to feel comfortable discussing it in great detail.
Basically, though, the acousticians do have this down to a science, or think they do, anyway. In general, you want the sound to go out towards the audience, but you also want some of it coming back. And you also ideally want the musicians to be able to hear each other well across the stage, too. There are some great halls where it sounds amazing in the audience chamber, yet the musicians have a hard time hearing across the stage. There are also some that sound great in the audience chamber, but too much noise also comes back from there to the stage. There are few that get everything absolutely right, and these are not necessarily the relatively newer ones. Symphony Hall in Boston is definitely one of the very best, as is Carnegie Hall in New York, to name two old school examples that still sound just as good or better than anything built lately. But, just like with audiophile equipment, there will be huge disagreements about which halls sound the best.
