Answer: Baking soda will counteract acidity, but too much can add its own "off" flavor. More importantly, it may not be safe to use in canning. The following paragraph is taken from a food safety web site (http://www.foodsafety.org/ ):
"...USDA recommendations state that tomatoes must be acidified using citric acid or lemon juice so it can be processed in a boiling water bath canner. It is especially important to acidify when tomatoes are fully to overripened or were allowed to sit off the vines several days before canning. Sugar may be added to reduce tartness."
Here is the page that instruct on canning tomato juice. http://www.foodsafety.org/he/he172.htm
Normally, I'd suggest you try different varieties--especially some of the older, hierloom varieties reputed to have the best taste. But it sounds like you've been experimenting on that front. Be sure you are using a non-reactive saucepan to cook the tomatoes--stainless steel or enamel, for example. Aluminum and cast iron pans can impart an "off" flavor. I hope this helps.
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