Proxy voting advisers Glass Lewis and Institutional Shareholder Services (ISS) have suggested Ritchie Bros. Auctioneers shareholders vote down the proposed acquisition of U.S. auto retailer IAA, a $6-billion merger that has pitted activist investors against each other.

In a report viewed by Insightia, Glass Lewis was skeptical that the combined company could deliver the projected synergies and said the transaction may ‘‘ultimately destroy value’’ for existing Ritchie Bros. shareholders, adding that the company remaining independent could be ‘‘the better risk-adjusted’’ scenario for those investors.

ISS raised similar concerns about the deal, saying in a note reviewed by Insightia that Ritchie Bros.’ “strong standalone prospects, proven over a period of time through robust performance, offer a better understood and verified path to shareholder value creation.”

IAA’s stock fell 7% Monday to $38.32, more than $6 below the cash-and-stock takeover offer. Ritchie Bros. shares closed up 3.1%.

Ritchie Bros. was quick to react to the verdicts, saying in a statement Monday that the combination ‘‘is expected to unlock substantial additional value that neither Ritchie Bros. nor IAA could achieve on its own, and we are confident in our ability to realize it.’’ The Candian company sees between $350 million and $900 million in total adjusted EBITDA growth opportunities in the deal.

The voting recommendations from Glass Lewis and ISS are a boon for Luxor Capital and other investors pushing against the deal, which is slated for a vote on March 14. Eminence Capital, Deep Field Asset Management, and asset manager Janus Henderson are also opposing the merger while activist firm Ancora Advisors, Independent Franchise Partners, and Eagle Asset Management are among its backers.

In a statement Monday afternoon, Ritchie Bros. 4.2% holder Luxor said the Glass Lewis and ISS reports are a ”clear rebuke” of the strategic rationale of the merger, adding that it now expects investors to ”overwhelmingly” vote against the ”flawed and ill-advised” tie-up.

Ancora, a 4% shareholder in IAA, recently took a small stake in Ritchie Bros. and issued several materials slamming Luxor and accusing the rival activist of trying to derail the deal to make a profit from an alleged short position in IAA.

Starboard Value joined the fray in January with a $500-million investment deal with Ritchie Bros. that some investors including Luxor called out for shortchanging the Canadian company’s shareholders. In its note, ISS said there are “lingering questions” about the Starboard deal.