NEW YORK, FEB. 8 -- A management-led group is offering to buy J.P. Stevens & Co. for $666.5 million, the company said today, but the textile giant's stock soared on speculation that higher bids would emerge.

The management group said it would pay $38 in cash plus debt securities it valued at $5 for each of Stevens' 15.5 million outstanding common shares.

In nationwide trading of New York Stock Exchange-listed issues, Stevens soared $12.25 to close at $45.62 a share.

The company said a special committee of its seven outside directors would study the offer, and Stevens might postpone its scheduled March 1 shareholders meeting as a result.

Some analysts suggested the management group would have to sweeten its offer, and they estimated that the company was worth closer to $50 a share based on its potential cash flow.

"I think the offer is entirely unfair to stockholders," said George O. Zimmerman, who watches the company for the securities firm Gruntal & Co. "This company has the capability of earning $4, even $5 per share, given some perk in the economy."

Leading the management group is chairman Whitney Stevens, a member of the family that founded the company in 1813.

Stevens for some time has been the object of takeover speculation, partly because of a major restructuring intended to make the company less vulnerable to stiff foreign competition in the apparel trade.

Beginning in 1984, Stevens moved out of most apparel fabrics and shifted its manufacturing focus to so-called home fashion textiles, such as sheets, towels, carpets, curtains and draperies, under such trade names are Laura Ashley, Ralph Lauren and Pierre Cardin. Those products make up 61 percent of Stevens' current business, with the other 39 percent including automotive products, yarn and industrial fabrics.

The restructuring made Stevens "a much more simplified company with a cleaner balance sheet," said Pamela Singleton, an analyst at Merrill Lynch Research.

Takeover speculation heated up last August following reports that a group led by Arthur Goldberg, president and chief executive of International Controls Corp., had acquired a 4.9 percent stake in Stevens.

Stevens said the latest offer by the management group was conditioned on a determination by Stevens' board that the offer was fair, approval by a majority vote of Stevens shareholders and receipt of the necessary financing to complete the leveraged buyout.

In a leveraged buyout, investors pay off the debt from the purchase with the cash flow of the target company or by selling some or all of its assets.

The group's offer also would establish an employee stock ownership plan that would give a stock interest in the company to nonunion salaried employees and hourly employees in nonunion plants.

The management group indicated it had commitments for $420 million in financing from Chemical Bank and Bankers Trust Co. and was "highly confident" it could raise another $276 million to complete the takeover.

In the fiscal year ended Oct. 31, Stevens earned $56 million, or $3.29 per share, on sales of $1.6 billion. This compared with $53.7 million, or $3.01 per share, on revenue of $1.68 billion in 1986.