Hm. The aftertaste is odd. I have a veritable ton of Adagio samples of all the different varieties and have never noticed a weird, non-tea-like aftertaste. In fact, one of the worst aftertaste offenders I've tried was a Teavana blend.
Based on that, I'd pin the aftertaste culprit to either your water or the Trinitea. If you're using tap water, you may want to run it through a Brita or something before making tea. Chlorine and things can make the tea taste off. I'd also give the Trinitea a thorough cleaning.
As far as the temperatures go...it's all really just ball-park. I think Adagio tries to give an easy approximation and stress that greens, whites, and some oolongs, shouldn't be brewed with boiling water. A lot of new tea makers don't make that distinction. Teavana, on the other hand, tries to present an image of being complete experts--their shop employees are actually called 'teaologists.' I actually disagree with a few of the Teavana temperature guidelines...but really, that's all they are--guidelines, not rules.