NEW YORK/BOSTON (Reuters) – Some top shareholders of IBM, disappointed by 11 straight quarters of falling revenues, are seeking help from activist investors to shake up the company, but have been turned down by both Bill Ackman’s Pershing Square and Jeffrey Ubben’s ValueAct, according to people with knowledge of the matter. International Business Machines Corp (IBM.N) is concerned about a possible attack by prominent activist hedge funds, and is working with two investment banks to formulate a defense plan, according to the people, who declined to be identified.