So I spent some time this weekend reposition my antenna. My pre-amp just shipped to day so I'll probably get it by the end of the week.
After studing TV Fool analysis carefully I noticed that I would got gain any advantage by going up 10 or 20 ft in height, so I tested antenna position around the eaves of the house. A also followed ADTech suggestion and point my antenna 8 degrees up.
I found a position to improve my Channel 40 (NBC) to 27 dB. The rest of the channels drop a little bit but not noticeable.
But channel 6 and other channels in the same direction 153 degree true azimuth
28
22
23 (my channel 6)
32
29
are basically gone (maybe I get 5 dB for them).
Once I got up there again to install my pre-amp I might try a few more position, but I have to cut my loses and live without channel 23 (6).
I let you guys know and thanks for your advice so far.
A very good source that I found and it is mentioned a lot in other threads is
www.hdtvprimer.com