I'll offer a link...
http://www.soundonsound.com/sos/nov00/articles/ustandingmons.htm
... which doesn't directly address this issue, but does explain some of the physics without maths - axial frequency response etc.
It is all a matter of acoustics, a science that most of us ignore, despite it being at the root of what we're about.
Speaker manufacturers basically want us to use their product, so are not going to tell you can't use them how you want to if another brand does say you can. KRK boldly make theirs a shape that you wouldn't want to put on its side - saves them telling you not to. What about all the designs with rear bass ports? How many of those say in big letters on the box - "Do not use against a wall"?
Yes, you see speakers on their sides on the meter bridge in pro studios, but commercial music isn't made market ready there and they place them so because it's practical and convenient. Also, to impress clients, they have high quality soffit mounted "mains" to demo over. The next stop for the music is
the mastering suite where you certainly will not find speakers on their sides, nor will they be atop a console, shelf or desktop. The mix engineer only has to get a balanced mix, the mastering engineer will fine tune it later and needs as much monitoring accuracy as possible.
Most home/small band/hobby studios though are having to do the whole process to the finished CD or Mp3, so I'd suggest it pays them to take monitoring accuracy a little more seriously. Your needs are closer to the Mastering Engineers than those of the studio mix engineer.
The business of on/off axis response and driver coherence and cabinet diffraction and early reflections etc, etc ... is certainly all somewhat technical. It is ommited from most product owners manuals because it is considered too difficult to explain and in a dumbed-down world, who wants to know?
If you do read up on the finer points of speaker design, positioning and the impact of room acoustics, you may reach the same conclusion I have - Speakers are pretty much helpless creatures - you cannot just expect accurate reproduction whatever you do, no matter what the specs were or how much they cost. Your room and the speakers position in it have to be good and these are factors more significant to monitoring accuracy than anything else. Ignore the physics if you like, its a free choice.
Given an acoustically good room, the narrow notches in frequency response cause by off-axis response errors, console reflections etc, are not as bad a problem as a measured response test might indicate - it doesn't in practice prevent getting a good mix. The room is the clincher - pro control rooms are acoustically designed to sound good. If your room is not like this, then don't expect doing what the pros do to work that well for you also.