Everyone has their opinions on fuel stations, but I have not had good luck with Shell in general.
Not sure what your options are, but I have good luck with Chevron, BP, Amoco, Texaco and Mobil.
FWIW, you're not the first person on here to have trouble with 91 octane fuel and an Unleashed tune. I don't think he quite accounts for how poor the 91 octane fuel is.