DENVER keeps on building: The largest airport in the United States is scheduled to extend its runways to international air traffic in 1993. Last year an enormous convention center began hosting conventioneers from all over the country. In 1995, Denver's spectacularly expensive new baseball-only park will flood with sports fans. And on Nov. 1, the Temple Hoyne Buell Theatre opened its brass-gilded glass doors to a 10-day inaugural extravaganza, the first night of which was broadcast nationwide over Arts &Entertainment Network. The Buell completes the Denver Center for the Performing Arts (DCPA) complex that features nine theaters and is second in seating capacity only to New York's Lincoln Center. But why does a relatively small community like Denver (with 1.5 million people), need so large a theater complex? It's part of broad efforts to revitalize Denver's economy. "Denver did not have a stage of a sufficient size and quality to accommodate the needs of touring productions available to it," says Charles Ansbacher, executive director of the Foundation for the Performing Arts Complex. Nor did Denver have a stage adequate to the needs of local producers either. The Buell holds 2800. For highly competitive bookings like "Phantom of the Opera" (opening soon at the Buell for an extended run) a theater with large seating capacity is necessary. The $36.5 million Buell project will also meet the needs for large local companies, according to Mr. Ansbacher. Opera Colorado, which has always performed in the round at Boettcher Concert Hall (part of the DCPA complex), will now have an alternative space with a proscenium stage. The Colorado Ballet will have its very first adequate space. The Colorado Symphony plans to use it as an alternative space when the opera is using Boettcher. The new theater is joined to the four other buildings that make up the DCPA. The new building is a huge structure with an 80-foot ceiling graced by a thunderbolt skylight in the lobby, six stories from ground level to the highest balcony. Though the lobby is rather ugly, its glass facade brings the outside in and gives a cohesive feeling to the entire complex. Aside from a bizarre use of neon lighting as decoration, the interior of the theater, with its blue-green carpets and its marvelous Colorado sands tone walls and proscenium, is remarkably attractive, warm, and elegant. A great deal of consideration also has been given to seating comfort for handicapped as well as other patrons. Local artists point to the Colorado sandstone used for the interior of the auditorium as one of the important factors in producing the fine acoustics: "Sandstone can both absorb and reflect sound," says Tom Gleason of the DCPA. "Because of its uneven surface and its porous nature it produces a balance of acoustical properties that appears to make it perfect for the purpose." "I was thrilled with the acoustics," says Ansbacher, himself a conductor. "During [Opera Colorado's] 'Otello' the other night, I found the sound crystal clear. I found the bass sounds very resonant and full, the treble very clean and clear. One could hear the singers better than in most halls." [But for this writer, sitting in two other sections of the hall, the sound was less than perfect. The hall will require more work.] The Denver political and arts establishments hope the complex will help make Denver the entertainment capital of the West. Colorado is heavily dependent on the 18-20 million tourists who visit the state each year. With the new airport, ballpark, convention center, an amusement park, and now the completed DCPA, Denver expects to lure many more visitors down from the mountains.