Tornante, run by Michael Eisner, former chief executive of Disney and buyout firm Madison said in a statement that the $378m price was "more than a full and fair price" for the baseball card company.
"If Topps shareholders feel differently and vote against our deal this week, we wish them well, but our price is final and we will not increase it," the investor group added.
Shareholders are due to vote on the deal on Wednesday. A number of proxy advisory firms have advised investors to vote down the deal, including Proxy Governance, which argued the deal did not appear to place a fair value on Topps.
Rival sports trading card company Upper Deck had made an offer for Topps that valued the company at $10.75 a share, but dropped it, accusing Topps of putting up roadblocks to its offer.