My guess would be cross breading with native species. Or at least, that is why they stopped stocking certain areas in RI.
Tigers are sterile aren’t they? Besides NH has virtually zero natural reproduction, and Browns, rainbows, landlocked salmon are not native to NH. It would be pretty hard to screw up the brookie genetics anymore than they already are. Everything in NH, minus a few tiny areas, has been stocked and therefore has compromised the gentics of whatever natural fish where there.
i havent noticed a difference in fight between species of trout. i have caught a bunch of tigers up to 22in long and have not seen a difference in fight between brookies browns and rainbows. it all depends on the size. a 20in trout fights better than a 12in trout. tigers especially male tigers are more pleasing to look at though.