I found the post with your tvfool analysis.
How are you pointing the antenna now?
It looks like channel 18 is about 35 dB stronger than channel 17. If you have your antenna pointed directly at channel 17, the 129º separation between the two stations should knock down this difference somewhat. The power difference, as seen by your receiver might only be about 15 to 25 dB.
The adjacent channel rejection will vary from one receiver to the next. A strong adjacent channel may or may not be the cause of reception problems on channel 17.
Are you sure that adjacent channel interference is giving you problems? How are you able to tell?
If adjacent channel interference is the main cause of your problems, there are probably alternate things to try before resorting to antenna doubling. Most of your channels seem strong enough such that you shouldn't need the extra 3 dB gain of antenna doubling.
1) Try rotating the antenna. Even if the main lobe of the antenna pattern is not aimed exactly at channel 17, you might be able to get channel 18 into a deep null in your antenna pattern, making the adjacent channel power difference much smaller. If you can drop the power of channel 18 enough, then your receiver shouldn't have any problems with channel 17.
2) Try different antennas (if you have that luxury). Other antenna designs may have deep nulls in different spots in the antenna pattern. If your current antenna happens to have a strong side lobe at 129º off the main lobe, then you might see better results from another antenna that happens to have a null in that direction.
Before jumping into this and spending lots of time and money on it, it would be nice to know if adjacent channel interference is really a problem or not.
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Can you recommend a lower noise pre amp? I understand there is one manufactured in the UK that is excellent but it is prone to catching fire. Don't want that to happen if at all possible.
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http://www.researchcomms.com/ is based in the UK and does make low noise amps. I haven't heard much about them catching fire, though.
http://www.kitztech.com/ also makes low noise TV amps and are based in the US.
Note that the amps from either of these companies do not come with weatherproof enclosures. If you plan on installing these outdoors close to the antenna, then you must provide your own weather protection.