The transition between dip and main beam is a notorious weak spot on the Tenere. The dip has a sharp cut off and the main is more like a pencil beam so leaves a large dark and unlit area right in front of the bike. If you adjust the headlamp unit up to improve the dip beam cut-off the result is an even larger dark void on main. You are experiencing a pretty normal Tenere problem!
There is a solution and that is to wire the the headlights so the dip beam stays on when main is selected. It is a dead easy mod to do and gets rid of this infamous "black hole" immediately in front of the bike on main beam. It is well worth doing and costs a few pence/cents, basically the the price of a Scotchlok!
Simply locate the rubber boot on the right hand side that protects the headlamp loom connectors, just above the radiator, and bridge the blue/black wire and the green wire. It is possible to just do with the tank in situ if you've got small hands and patience, but is far easier with it removed.
(The image shows radiator and tank removed - you don't actually need to remove either. It just so happens I did the mod and took the picture when the bike was stripped back to do the valve clearances.)
These two wires can also be soldered together in the switch as an alternative.
One cautionary note though: This mod is only really recommended for a maximum of 55W bulbs in both main and dip. It is inadvisable to use 100W bulbs for example, as the all the current is carried in the green wire for both lamps. The cross-sectional area of the the headlamp wiring is gives a rating of 16A (even though the fuse is actually 20A!), so the theoretical safe maximum is 192W combined (although it will be slightly more if the engine is running and at peak alternator output). However, bear in mind 192W is using up ALL the spare capacity of the alternator.
FYI, this thread covers the Tenere's alternator output and spare capacity:
http://www.xt660.com/showthread.php?t=18294
This post shows pictorially how to do the dip beam mod in the switch:
http://www.xt660.com/showpost.php?p=192435&postcount=9