Updated from 7:33 a.m. EDT

The bad news just keeps coming for



, the biggest publicly traded

New York Stock Exchange

specialist firm.

Early Friday, the firm said the head of its specialist unit, Robert Murphy, is quitting, but swore the move has nothing at all to do with the ongoing investigation of floor trading at the Big Board by the NYSE and the

Securities and Exchange Commission

. Then late in the day, LaBranche said it received a subpoena on Oct. 30 from the SEC in conjunction with that investigation.

All in all, it added up to another bad day for LaBranche, which saw its stock fall 88 cents, or 9.6%, to $8.31.

Over the past four months, shares of LaBranche have slid 51% in the wake of the investigations, a poor third-quarter earnings report and the controversy over the hefty $139.5 million pay package awarded to former Big Board chairman Richard Grasso.

In a regulatory filing on Friday, the company said it continues to believe it acted properly in carrying out its duties as a specialist. Notwithstanding that claim of innocence, the NYSE has notified LaBranche that it faces at least $43 million in potential liability for the trading violations.

In that same regulatory filing, LaBranche also confirmed that the SEC began its investigation on Oct. 17, a day after the NYSE announced that it intended to bring disciplinary actions against LaBranche and four other leading specialist firms.

The SEC opened its on investigation because it was dissatisfied with the pace of the NYSE investigation. In fact, the NYSE seemed to be willing to impose lesser penalties on LaBranche and the other specialist firms, until the SEC prodded the Big Board to take a tougher stance.

Meanwhile, Murphy resigned both as a director of the parent and as chief executive of LaBranche & Co. LLC, the company's NYSE specialist firm. He'll be succeeded at the subsidiary by Alfred O. Hayward, currently executive vice president of LaBranche & Co. Inc.

"My decision to leave LaBranche, which is not related to the ongoing NYSE and SEC investigations of LaBranche's specialist trading practices, is a difficult one," Murphy said. "I have enjoyed my career at both LaBranche and Robb Peck McCooey Specialist Corp., but I believe the time is right for me to pursue other interests."

Back in August, just before the Grasso pay scandal broke, Murphy resigned as vice chairman of the NYSE's board of directors. Murphy's resignation came at a time that LaBranche and the NYSE were locked in a bitter dispute over access to some of the trading firm's internal emails. Ultimately, LaBranche lost that battle.

The current 21-member NYSE board is being phased out in the plan by John Reed, the Big Board's interim chairman and chief executive, to overhaul the exchange's corporate governance policies. The new board will have just eight members.