While on-axis tweeters may measure flattest for treble, they can sound too bright, especially if you sit close to the speakers (air absorbs treble much more than other frequencies = sit closer to the speakers and the treble is relatively brighter).

The fastest way to move the tweeters off axis is to tilt the speakers/stands back a little so the tweeters are aimed over your head. You'll get a little more high frequency roll-off and as a bonus, the soundstage may move higher (singers may sound like they are standing in front of you singing rather than sitting across the room from you singing).
I do this at home. You may need a few weights on the front of the base of the speaker stands to stabilize them after tilting them back. My stands had four spikes in them -- I made the front spikes as high as possible and removed the rear spikes = stands now tilt backwards aiming tweeter over my head = more mellow high frequencies (needed mainly because I sit quite close to my speakers.