ONE OF THE basic assumptions of American concert life - that the best concert halls are long, narrow, rectangular boxes - is being challenged in Denver, where Boettcher Concert Hall, the first "surround" hall in the United States, opened last month.

At the very least, the new $13.2 million home of the Denver Symphony Orchestra is a daring experiment that compensates for its shortcomings with a great deal of unorthodox charm. At best, it may represent the wave of the future in American concert hall design - and judging by the salvo of critical bravos that followed the inaugural concert on March 4, there are many who share this opinion.

In a surround hall, the state is more or less in the middle of the room rather than at one end, and the audience literally surrounds the orchestra. Boettcher's stage is off-center, so most of the hall's 2,719 seats face the state in the traditional way - that is, so that the patrons look at the conductor's back. But there are nearly 600 seats behind the stage and many more that offer side views of the orchestra.

The advantages and disadvantages of such a design may be easily imagined. One of the chief advantages is intimacy; because there are fewer rows of seats, nobody in Boettcher has to sit more than 85 feet from the stage, and 80 percent of the audience is within 65 feet of it. One of the main disadvantages is tonal imbalance; the seating arrangement of the modern symphony orchestra presumes that the audience is "out front" and is expressly designed to throw sound in that direction only.

On the basis of hearing two performances of Beethoven's "Missa Solemnis" at Boettcher in mid-March, I'm inclined to take a conservative view of its importance as a precedent.

That Denver has acquired a concert hall of which it can be justifiably proud is beyond argument. But whether this or any surround hall is a genuine improvement over traditional rectangular facilities like Boston Symphony Hall and the Kennedy Center concert hall is highly debatable. At Boettcher I sat in four different locations, including the section directly behind the choir and balconies at stage right and left; but it wasn't until I moved out front to a seat from which I faced the soloists, as one does in a conventional hall, that I heard an entirely satisfactory balance among soloists, choir and orchestra.

There, however, the sound was expectionally clear, warm and resonant - and it must be remembered that most of Boettcher's seats are so situated.

Boettcher was designed by the New York firm of Hardy, Holzman, Pfeiffer Associates (the firm that designed Minneapolis' Orchestra Hall) in collaboration with acoustician Christopher Jaffe of Norwalk, Conn.

The individual most directly responsible for the hall's unconventional shape, however, is Brian Priestman, the red-beared music director of the Denver Symphony Orchestra.

When Priestman arrived in Colorado in 1970, he made it one of his first orders of business to persuade the orchestra's board of directors of the need for a new hall to replace the old, acoustically lifeless Auditorium Theatre. And he made it clear that he wasn't interested in leading the musicians into another formal, rectangular shoebox.

"I said from the first that one thing had to happen if the orchestra here was to grow: ther had to be a new hall," Priestman said.

"I downed an awful lot of martinis in convincing people what kind of hall Denver should have."

Convince them he did, however, and in 1972 Denver voters approved a $5-million-bond issue for a new concert hall, with the remaining revenue - then estimated a $6 million - to be raised privately. The hall was originally scheduled to open in 1976, Colorado's centennial, but various delays took their toll. So did inflation: Boettcher has cost about $2 million more than planned, and even at that the orchestra board was forced to sacrifice a rehearsal hall and many lobby amenities.

From the outside, Boettcher is an unimposing brick box that was clearly designed to complement, rather than overshadow, the older brick buildings nearby. Its light brown brick is carried into the lobby, a cramped space that seems even smaller than it is because the space is broken up by exposed pipes and ducts and metal stairways and catwalks leading to the balcony seats. This pseudo-industrial decor is Boettcher's least attractive design element; fortunately, it is left behind when one enters the auditorium proper.

The music room is not so much a circle as an oval. And a very active oval it is; on both sides of the large, oak-floored stage, steep balconies swoop down from the upper reaches of the room. The front edges of these balconies are made of gray plaster molded into a continuous series of undulating waves and decorated with two thin horizontal stripes of gold leaf - Hardy, Holzman, Pfeiffer's playful reminder that Boettcher is a complete break from the opulent concert halls of the past.

The seats themselves are made of steam-bent plywood. Their cushions, which are covered with a mohair fabric of an orange shade offically described as persimmon, are firm but comfortable. The seat backs arr unusually high, in most cases extending well above the heads of the people sitting in them - the idea being to give them the same acoustical tendencies whether occupied or empty. The high backs pose no sightline problems, however: Boettcher's floors are as steeply raked as those of a football stadium, and everyone has a clear view of the stage.

Unfortunately, everyone also has a clear view of the ceiling, which is hopelessly busy. In a surround hall, there's no simple way for reflected sound to reach the musicians themselves so that they can all hear one another clearly. Jaffe has attempted to resolve this problem by hanging a galaxy of large translucent acrylic discs from the ceiling. There are more than 100 of these discs, and they're an enormous visual distraction. The stage lights are affixed to dark green catwalks - also suspended from the ceiling, as are, of course, the four "floating" seating rings.

Nevertheless, Boettcher impressed me as a hall whose defects are more than balanced by virtues - not the least of which is its lively insistence that everyone who enters rethink his or her notions about what listening to a concert should be like.

The musicians' first reaction likewise seems to be favorable. One of the "Missa" solosists, soprano Phyllis Curtin, told Priestman that singing in Boettcher is like singing in a salon.

"And that really thrills me," he said, "because that's exactly what I wanted."

Roy M. Close is the music critic of The Minneapolis Star.