We consider a revenue-maximizing single seller with mitems for sale to a single buyer whose value v(·) for the items is drawn from a known distribution Dof support k. A series of works by Cai et al. establishes that when each v(·) in the support of Dis additive or unit-demand (or c-demand), the revenue-optimal auction can be found in poly(m,k) time. We show that going barely beyond this, even to matroid-based valuations (a proper subset of Gross Substitutes), results in strong hardness of approximation. Specifically, even on instances with mitems and k ≤ m valuations in the support of D, it is not possible to achieve a 1/m1-ϵ-approximation for any ϵ>0 to the revenue-optimal mechanism for matroid-based valuations in (randomized) poly-time unless NP ĝ† RP (note that a $1/k$-approximation is trivial). Cai et al.'s main technical contribution is a black-box reduction from revenue maximization for valuations in class V to optimizing the difference between two values in class V. Our main technical contribution is a black-box reduction in the other direction (for a wide class of valuation classes), establishing that their reduction is essentially tight.