I suggest that you are doing this backwards. Pick your can first. The best and minimally required amp to properly drive the TH900 is very different from those to properly drive the HE-6.
Once you decide on your can (there are multiple threads devoted to each individual choice you have listed and to it alone), then you can go to the Summit-Fi section of the forum and/or the Headphone Amp (full sized) sub-section dedicated to each individual amp and join the discussions there as well.
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