That's your first thing to try? I think that's insane. Again, it's extremely unlikely the speakers have anything to do with the "tipped up" sound. As far as I know no one has reported them having anything but flat pretty flat FR, with measurements.
He's not an expert crossover designer, either, to know when as many problems might be created as solved by changing something.
I assumed the owner has already tried (without success) to re-position the speakers. Changing the tweeter level is a very reasonable thing to try and is very effective. It is not making the speaker inaccurate at all. Many manufacturers (like Wilson Audio) have user-adjustable tweeter levels for very good reasons. Yes, speakers should measure flat in an anechoic chamber, but that should be just the starting point. The final response in a real room should be set by adjusting the tweeter level by ear to what sounds natural (to your ears and in your room).