Where does fatique come from?

As covered by folks above, fatigue is mainly caused by brightness especially in the midrange to high frequency range. Sometimes there are simple solutions that can ameliorate on this "fault" other than swapping equipment or speakers. Two effective way out is in speaker placement and room treatments. Room treatments will greatly enhance the sound system while eliminating any deficiencies such as sharp piercing treble(which results in fatigue) or bass boom in the low frequencies etc. Also, you might be surprised that a simple speaker positionining may eliminate fatigue altogether by a minimum or no toe-in speaker placement.

Yes, direct sound to the listening position is certainly not too good. With the drivers directed away from the listening position especially the tweeters, the ears will not be subjected to direct sound that may cause fatigue. One may also be able to crank up the volume higher without hurting the ear-drums with some proper speaker placement especially with minimum toe-in. In this sense, I suppose that omni-directional speakers may hold an advantage although I have no experience with Ohm Walsh designs.