OK, good on the filter. The pickup sock in the tank could be plugged as well as the trash filters in the carb inlets if it has them. The obstruction in the tank maybe a splitter in the lower end of the tube. Try a smaller hose and twist it around some.
If the yellow gas was a piss yellow tint, then I'd agree on the 'going skunky'. Other yellow tints may be OK. If going bad, it would also have a distinct acrid odor to it beyond the normal gas smell.
BTW we run Shell 93 all the time in my son's 340.... runs great.