"There was a renovation spirit going on at Lincoln Center," she said.  Ms. Moss and her colleagues hope that the novelty of the new setup and the experience of sitting so close to an orchestra will attract new audiences, the current mantra of classical music presenters.  To that end the stage seats are the cheapest in the house: $38 tops, compared with $58 for the most expensive seats.  To the practitioners and purists, of course, the most important issue is the effect on the sound.  At a rehearsal this week, several of the musicians in the orchestra spoke enthusiastically of the acoustics, although some had reservations, at least with the current position of the canopy and no experience yet with an audience. Bodies in the seats make a large acoustical difference.  "This hall used to seem so impersonal," said Ron Oakland, a violinist. "Now there's a general warmth in the sound."  Michael Gillette, a member of the second-violin section, which sits to the conductor's right, said he was hearing himself better in the section but still had some trouble hearing the first violins across the stage. "You actually feel more like you're doing something the audience can respond to," he said. "That's the essence of live music."  The festival's music director, Louis Langrée, repeatedly jumped off the stage from the podium -- located roughly where the second row of seats would normally be -- and ran into the seating areas, listening with his arms akimbo.  In an interview later, he said the new setup created more resonance and presence in the music.  "You don't have to fill the house with volume," he said. "You can really just concentrate on energy without being anxious that it will be too anemic or too thin."  The smaller stage seems more "organic" for a chamber orchestra, said Mr. Langrée, a Frenchman who has often conducted on stages surrounded by seats in Europe.  "In two or three years, I will not be afraid to program, with this size orchestra, Beethoven's Seventh," he said. "It's the energy that you need, the manic energy, not the volume, the loudness, the decibel."  The stage, made of a South American hardwood literally called blood wood, extends out 30 feet. Eleven rows of seats were unbolted from the floor and are stored under the new stage, which is to be removed at the end of Mostly Mozart's season. Three new rows of seats line the sides of the stage, and another section rises in the back. Low walls with railings separate the stage seats from the players. Cylindrical silk light-shades have been hung over the stage to create a homier appearance.  Audience members are close enough to read the music on the stands, or to tap Mr. Gillette on the shoulder.  Sitting in the close rows of the stage seats will be a visceral experience and, depending on the location, cause some sections of the orchestra to be heard louder than others.  A listener in the left side seats at Tuesday's rehearsal heard a French horn growl out low notes and saw a flute's silver glint. From the rear seats, a clarinet player was seen swabbing his instrument. On the right, foot soles vibrated with the thump of the timpani, and the fingers of woodwind players were seen fluttering through trills. The platform shook with each Langrée leap.  The transformation cost about $575,000, which mostly came out of Lincoln Center's capital budget, and reduced capacity by 365 seats, the festival said, to 2,363 from Avery Fisher's usual 2,728 places.  Ms. Moss said revenue would be lost at some concerts because of the lower capacity, but the festival hopes to gain from larger audiences in the long run.  The New York Philharmonic has said it would seriously consider moving the stage in future plans to refit the hall. Last year it tried out a simplified version of the stage extension during a rehearsal.  "It's a great next step from the experiment with the thrust stage a year ago, and hopefully what the future will hold," Eric Latzky, the Philharmonic's spokesman, said of the new installation.  Stage seating is not unheard of in New York. Chairs are often put behind a soloist, and performers and audiences are occasionally reconfigured for specific concerts.  The semipermanent nature of the Mostly Mozart arrangement (it will be used for 17 concerts) required complex logistics. Ushers had to be trained in how to seat people and box-office employees instructed on the new configuration. Space had to be left so pianos could be moved on and off.  Staff members struggled with the problem of renumbering the seats. When they realized that the seat numbers were on metal plates, they had the idea of applying magnetized labels with new row numbers over them.  Mostly Mozart officials proudly described the solution but several days later expressed fears that people would steal the labels if the information became public, and declared they would be taped down.
Article on changes made to Avery Fisher Hall for Mostly Mozart Festival, which opens tonight; stage has been extended 30 feet into audience and seats have been added on sides and behind; it is only major stage in city to have such arrangement for orchestra concerts; photo (M)