Not sure about the DB8, but the DB8e with both panels pointed the same, is more directional than the 91XG.
If you need a narrower reception field, you can stack yagi antennas. Vertical stacking narrows vertically, and horizontal stacking narrows horizontally. You can stack in both axes and narrow in both. Look here -
http://www.ifwtech.co.uk/g3sek/stacking/stacking2.htm - orjust google "stacked yagi."
Take a look here
http://www.hdtvprimer.com/antennas/siting.html especially the sections "Trees and UHF" and "Skyline Multipath." Moving the antenna up/down or to another location on your roof might help.