nagaimo and dashi questions

Maki, I have just come upon your site for the first time today, when I was googling 'nagaimo.' I quickly read a number of your informative pieces and recipes, and I am soooo impressed by your articulate well infomed writing style and your very helpful user-friendly attitude. I have a few questions and so appreciate your help. When i purchased nagaimo today at my local asian market , there were long tubers for $4 lb. and flatttened blob shaped nagaimo for $17 lb. Plse help me understand what might be the flavor difference between those 2 nagaimo(that would explain their price difference?) I bought the nagaimo for yama kake. Are nagaimo and taro really the same thing or is it simply that 'yam' gets used to name scores of different tasting tubers? It doesn't resemble taro I have worked with! Also, one of the nagaimo recipes i googled mentoned treating nagaimo to a short vinegar water bath to counter a chemical. Is that chemical the same that can cause itchiness and if so, is it countered just as well by wrapping the tuber around the outside when grating it- to avert skin contact with the tuber skin? When I make dashi and follow the traditional instructions which call for "bring to boil and turn off and.".........., the result is too weak for my taste. I end up doing the unthinkable and simmering the katsuobushi for 15 or 20 minutes, until the dashi has more flavor. I do not detect any bitterness from this. Am I crazy? Thanks much for your help.
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