I did an experiment with the horns on both my mesa cabs yesterday and my conclusion is that you dont get much out of them...
The only way i could get some proper sound to come out of them was by playing my 6 string bass's high C with the treble boosted on my amp and the tweeter control maxed!
On a jazz bass with half dead rounds all i got was a bit of sheen and clang when slapping or picking but nothing that was really life changing especially when playing fingerstyle with a normal EQ (without pushing the highs to stupid levels)...
Even then, its not really sound you hear putting you ear in front of the tweeter, it just makes your overall sound brighter when listening to it a few feet away...
I would say those horns are designed not to do much and dont worry about it!
I just set mine to 5k normal mode, cut the tweeter 2db just as a way to protect it from blowing if i push the cab hard (not even sure it makes a difference but i always read about people blowing tweeters so i thought id try my best not to do it!) and get on with it...