Again, I am not saying the problem is not a phase thing, it probably is, but I am saying with DEQ on, especially if plotted at low spl level, you will see a steeper slope (but not a trough) because the DEQ boost varies inversely with frequency and volume. They did it that way to match human hearing characteristics, that we are less sensitive to the lower frequencies at low spl. That's what make DEQ so much better than the simple "loudness" button in the old days.
Put it this way, a slope down towards the higher bass frequencies would be at least partially attributable to DEQ, Pogre can prove it simply by overlaying the DEQ on Vs off as I did in August 2014. If there is a trough, then it could well be a phase issue. The two things are not mutually exclusive, though the DEQ effect is normal and good, the phase effect, if resulted in a trough, is not good and can be fixed.