Cruzer, you are 100% correct about keeping the towbar level with the tabs on the hitch. I talked to Blue Ox when I was setting up my Rubicon to tow and they said as long as the towbar centerline is no more than 4" above the tabs on the toad that is within specs.
Here's what they state in the instructions for the Aventa LX:
It is important that the attachment
points at the center of the attachment tabs and
the center of the coach's hitch receiver should be
of equal distance. If a deviation must be present,
it should be no more than four (4) inches from the
tabs up to the hitch receiver. There should be no
deviation allowed for the hitch receiver tube to be
below the level of the attachment tabs.
Since I mounted the tabs fairly high on my bumper to further protect them from hitting a rock or other immoveable object I had to compensate by adding a Blue Ox drop hitch, which I reversed and made it into a lift. I checked this out with Blue Ox and they said it was fine in this configuration.
The tabs on my Jeep are about 8" above the centerline of the hitch. I decided to go with a 10" lift in case I later installed taller tires.
You were absolutely correct in stating that the deviation can't be in the other direction.
So far, I have towed +10K miles with zero problems other than I described in my earlier post about stupidly backing up with the Jeep attached.