A piano, the layman might think, is a piano, is a piano.

Not so, according to a group of enthusiasts who met for the last two days at the Smithsonian's Hall of Musical Instruments -- described by one of their less reverent members as "a bunch of nuts who go around playing old music on old pianos." Nuts or not, they made a good case for their basic thesis: When you hear Beethoven on a modern piano, you are not really hearing Beethoven.

There is not only a world of difference between today's concert grands and the pianos of Mozart's time, but also between a modern Steinway and a modern Boesendorfer, an 18th-century Broadwood and an 18th-century Dulcken.

The differences were brought out sharply in a heated exchange between Robert Winter, UCLA pianist-musicologist, and Harold Schonberg, New York Times music critic, author of "The Great Pianists," a basic book in the field, and ardent devotee of the modern concert grand.

Beethoven, argued Schonberg, would have liked a modern Steinway: "Beethoven went through his life complaining about the limitations and inadequacies of the instruments of his time. In his mind, he invented the romantic piano."

Winter strode over to an 18th-century piano and struck a Beethoven chord: "Listen," he said, "I can hear F, A-flat, C, F -- you can really hear a triad, all four notes. On the Steinway, you hear a blur." He walked over to the venerable Paderewski Steinway and struck the same chord, which immediately blended into one mixed sound.

"You have the pedal down all the way," shouted Schonberg.

"I don't have the pedal down at all," answered Winter.

But Winter wasn't doctrinaire about it. A day later, he was gesturing dramatically at the Paderewski Steinway and saying, "I'd give my right arm for that piano," apparently willing to accept a life of left-hand repertoire for one vintage concert grand. An ardent advocate of the fortepiano (the 18th-century Viennese name for the instrument), he has four in a small apartment and has trouble walking around them.

Winter and Shonberg were among a dozen experts convened to discuss the subject and to promote what they treat as a holy -- cause -- letting audiences hear piano music played on the kind of pianos it was written for, Concerts alternated with panel discussions, during which one or more panelists would often jump up to illustrate a point on one of the half-dozen old pianos crowded on the hall's stage or on the Paderewski Steinway, which sat in a corner.

(The 19th-century concert grand, given to the Smithsonian by the Steinway company, was used by Jan Ignace Paderewski on one of his American tours.)

The two-day colloquium on "The Early Piano," sponsored jointly by the Smithsonian, the Music Critics' Association and the Carnegie Hall Corporation, marks the second major attack on the grand piano in this century. cThe first, which began gaining momentum about half a century ago under the hands of Wanda Landowska, was on behalf of the harpsichord. Since then, other old instruments have been rehabilitated, and there is a brisk industry providing new recorders, flutes and viols for a growing market.

The colloquium attracted visitors from as far away as England (also a hotbed of forepiano enthusiasm), and the quality of the audience was almost as high as that of the panels. Among those sitting in the hall, listening and occasionally making a remark from the floor, were harpischordist Albert Fuller, who carried on the Landowska tradition to a growing younger generation, and violinist Sergiu Luca, who is one of the major living experts on the baroque violin.

The new wave of enthusiasts for the fortepiano are not rabid anti-Steinway enthusiasts.Malcolm Bilson, who has had a modern reproduction of the piano in Mozart's home built for him, also keeps a modern piano for modern music.

The pianos of most interest to fortepianists roughly coincide with the lifetime of Beethoven (1770-1827), the period of the instrument's most dramatic development, with a special turning -point around 1800 when its five octaves were advanced to six. The later 19th century added another octave, boosted the piano's power to fill the larger concert halls of the period, and began a period of small refinements which are still continuing.

The 20th-century taste for instrumental authenticity, which began recently with medieval, Renaissance and baroque music, has now reached the classical era, spurred by recordings from small, specialized labels aimed at a connoisseur audience, and by a growing number of performers who specialize on post-baroque instruments.

The idea is beginning to catch on, but Bilson evoked a character from the Peanuts comic strip to express the strangeness of it all:

"When I go out in a big hall with my little fortepiano," he said, "I still feel like Schroeder sitting down in front of his toy instrument."