Hi mOtion,
There were a few papers in the JAES that used measurement and listening tests which "concluded" that woofer position and orientation could be determined at 60Hz and above. The measurements looked at the upper bass harmonics, and the conclusion section discussed their effects on bass source location.
This matches the experience of one friend who built a narrow front MTM tower with opposing side firing woofers 3rd order crossed at 200Hz, and was very disappointed with male voice.
Some of the latest 3-way designs use modern speaker and crossover technology to put most of the human vocal range on one slightly larger midrange (Linkwitz 8" ~ 120-1,400Hz), and this design strategy would probably get the best results from side firing woofers. Magnetar and Lynn Olson are experimenting with 12" midrange.