Equalizing for a flat in-room response requires adding a high-frequency boost.

Evidence

The loudspeaker used was highly rated in double-blind evaluations and was commendably flat and smooth on-axis with other well-behaved spinorama curves. Such loudspeakers yield rooms with a gentle downward tilt, as shown in Figure 12.16. For these experiments it was equalized to produce a flat steady-state in-room response, which added a high-frequency boost that, possibly, listeners would recognize and correct. — Toole, Floyd E, Sound reproduction- the acoustics and…, 1353