"We are convinced that having an owner's perspective in the boardroom yields better results, that this board is in dire need of fresh insights, and that our candidates are more qualified than the company's emissaries we are seeking to replace," Loeb wrote in the letter released Monday.
"We are confident that adding three shareholder voices to this twelve-person board will do more to improve Sotheby's long-term growth and increase its share price than would rubber-stamping the company's latest set of hand-picked nominees."
Shareholders will formally vote on the new board at Sotheby's annual stockholder meeting May 6, but proxy cards were sent at the end of March, prompting Third Point to position itself early. Institutional Shareholder Services, which advises stock owners, will likely release its recommendation on the fight this week.
Third Point's director nominees are Loeb himself, plus turnaround expert Harry Wilson and jewelry executive Olivier Reza. Sotheby's nominees are private equity advisor Jessica Bibliowicz, retail mall developer Robert Taubman, and restaurateur Daniel Meyer.
Loeb's new letter compares the credentials of each director. For example, it says that eatery maven Meyer is essentially unqualified to advise on luxury art.
"Notwithstanding how much we would enjoy a board meeting catered by the Union Square Cafe, we respectfully submit that Harry's credentials are far better suited to board service at Sotheby's than Mr. Meyer's," the letter said.
Third Point declined to comment and Meyer was unavailable for comment. Sotheby's did not immediately respond to requests for comment. The company has aggressively rebutted Loeb's characterization of it as deficient.
Activist investor Third Point manages more than $14 billion and owns 9.65 percent of Sotheby's common stock as of Feb. 17. Loeb has agitated for change at the auction house since last year, causing Sotheby's to institute a so-called poison pill to protect itself from additional Third Point ownership.
Sotheby's stock price has fallen nearly 25 percent this year.