YOU CAN'T stop that guy," says a National Symphony Orchestra player. The tone in his voice is one of admiration, expectancy and not a little surprise; he is talking about Martin Feinstein, who has just become the president of the Symphony, recalling his first talk to the players after taking the new job.

Feinstein began by promising not to conduct anything -- a point that could not be taken for granted in his case. And he climaxed his talk by outlining his program: "My ambitions are very simple. I want to make this orchestra not one of the top five but absolute number one. And I want to finish every season in the black."

Unstoppability is one of Feinstein's keynotes; another is unabashed ambition. He is likely to need both of them in his new job with the symphony, which has been struggling with interlocking problems of finance and image (hopefully interpreted as growing pains) and has brought him in after deciding that it is time for a major change of managerial direction.

One of the problems was pinpointed late last year when Leonard Bernstein rushed into the dressing room of his friend, conductor Mstislav "Slava" Rostropovich, after a performance of Mahler's Seventh Symphony and ebulliently told him: "The National Symphony is World Class now." Bernstein had come to Washington to conduct the Vienna Philharmonic, which is a good example of what the elusive term "world class" means.

Comparing the two orchestras even the National's most ardent fans might conclude that Bernstein was . . . premature in his estimate. But it is a good orchestra getting better, and in recent years the average quality of its playing has probably outrun its reputation. Witold Lutoslawski, who has been in town the past week conducting his own very difficult music, says he "didn't expect the quality I found in that orchestra; I presented them with some extremely difficult problems, some totally new musical procedures, and they assimilated them much more readily than I had anticipated." He added that this unexpected ability was fortunate, because American orchestras are not usually given enough rehersal time for complicated new music.

The reason Lutoslawski was in Washington to conduct his Cello Concerto and discover the quality of the NSO is significant. He had written a new orchestral composition, "Novelette," for his old friend Rostropovich, for whom he had written the Cello Concerto 10 years ago. Rostropovich has brought in several prestigious commissions, but his international star status is based chiefly on his work as a cellist; his personal glory extends to his orchestra only occasionally, fitfully. The Feinstein appointment, one clearly gratifying to Rostropovich, may be a sign that this situation is changing.

While it has been growing musically, the orchestra has been falling behind at the box office. Subscription sales are down for the current season; and even if they were up, they cannot be a complete solution. The orchestra's current revenue from tickets is a bit more than 40 percent of annual expenses ($3.2 million out of $7.1 million); and if it sold every ticket for every performance, it still could not get over 50 percent. "We have been in a financial predicament almost hourly for the last several years," says board chairman Austin Kiplinger, "and in examining the financial problem we decided that it was just part of a total condition that called for change. We progressed from purely financial concerns to the entire problem of long-term orchestra management, and we felt that bringing in Martin Feinstein would be a strengthening move."

Feinstein has never been the president of an orchestra before; he has been a key member of the Hurok concert-management organization for a quarter-century, the executive director of performing arts at the Kennedy Center for eight years, and the managing director of The Washington Opera (a job that he will keep along with his National Symphony position) officially since Jan. 1.

When his NSO appointment was announced, there seemed to be some expectation that Feinstein's talents might be used for fund-raising. A week later, Kiplinger said, "I have changed my thinking on fund-raising; the board has the major responsibility for that and the staff should not be saddled with it, although we expect Martin to help us in approaching the public." Feinstein believes his basic responsibility, in collaboration with Rostropovich, is to help provide the board with an orchestra they can "sell" to corporate contributors, foundations and perhaps the government -- and Kiplinger agrees.

Feinstein's track record in Washington so far seems to relate more to creativity in spending money than in raising it. According to widespread conjecture, the size of deficits in his projects (in a field where deficits are a pervasive fact of life) is one reason why he and the Kennedy Center management are now going their separate ways. "I've been given credit for raising money locally," says Feinstein, "but it is not true -- that's all been done by Roger Stevens and Jullian Poole. Where I have gotten money has been from foreign governments, to assist the visits here of their opera and ballet companies. I don't think they are likely to contribute much to the National Symphony."

What can Feinstein contribute to the National Symphony? He prefers not to venture into job-description this early in his new career, but Rostropovich and Kiplinger have both formulated some expectations. "Within the policy guidelines set by the board and within the available financial capacity," says Kiplinger, "the president and his staff will see that the orchestra prospers and functions as it should. Martin Feinstein is acquainted with the Realities in the field and he knows music. In his previous work, he has demonstrated an ability to plan years ahead, and that is a talent we will be using. We knew that he has long had a close friendship and a relation of mutual respect with Slava, and we took that into account. Slava had made it plain that he wants some advice on programing, and Martin can supply such advice."

Rostropovich approaches the Feinstein job-description from a somewhat different angle: "I think what the orchestra needs is a person with a great many contacts and with whom contact is simple. He must keep in constant touch with the board. Our financial position has to be of great interest to him. He must be in close contact with the music director -- with whom, in this case, he has had long years of friendship. He has to understand how to balance a program, how to promote the sale of tickets, and all other questions related to the status of the orchestra. He should have a one-to-one relation with the members of the orchestra; each of then has individual questions, and his door must be directly open to them. It will be very good if he has a close human contact with soloists and conductors who may appear with the orchestra, and he should have contacts in other cities, here and abroad -- for example, to arrange for tours or the exchange of orchestras. I don't think we could find anyone better than Martin."

This could be Rostropovich's description of the job or his description of Feinstein -- the two are probably interchangeable. It would be logical to assume that Rostropovich was the one whoe secured the new job for his old friend, but all parties in the transaction say this is not so. "Neither of them initiated the move," says Kiplinger. "This appointment was not pressed upon the board, and Martin Feinstein did not seek the position. He was obviously interested and excited when we talked about it, but he did not come after us. We went after him."

"I did not solicit the job; I was invited," Feinstein said a few days ago.

"Naturally, I was pleased at the offer, but I did not accept immediately. They would have liked to have me full-time, and I told them that I would not give up the opera. They first approached me about six weeks ago, and when I refused to leave the opera they went away. Then, about two weeks ago, it started up again very actively. Once they accepted my decision about the opera, it was done in a few days. The executive committee had a two-hour meeting on Thursday the 17th, Austin Kiplinger told me the next day that they had voted unanimously to hire me, the full board of directors voted on it on the 22nd and the announcement was made right after the final vote." Kiplinger's version of the sequence of events is substantially the same.

Even if they did not make the appointment happen, however, Feinstein and Rostropovich are obviously a team capable of making other things happen -- musically and promotionally. Except for a few details (such as the opening gala benefit concert), the NSO's 1980-81 season already was mapped out and ready to be announced when Feinstein took office, and his impact will probably not be fully felt until the 1981-82 season -- coincidentally the final season in Rostropovich's current contract, which means he will have added negotiating leverage. That is also the season in which he has promised to spend 19 weeks with the orchestra, rather than the customary 12.

With principal guest conductor Rafael Fruhbeck de Burgos now committed to six weeks, that seems to leave no gaps in the orchestra's normal 25-week season -- though, of course, Rostropovich may spend some of his time here in that season as a cello soloist rather than a conductor. It would be surprising if Martin Feinstein's arrival were to coincide with a reduction in the number of big-name guest conductors -- this is contrary to all that is known of his past careers. It would be less surprising if the arrival Feinstein meant an extension of the orchestra's season, and perhaps a new venture into such areas as festivals, or the performance of operas in concert form -- a subject dear to Rostropovich's heart and a source of frustration to him so far in Washington.

But it will be a full year before the NSO divulges its plans for that season, and meantime the field is left open to wild speculation. That, too, is an activity frequently found in connection with Martin Feinstein.