Bob - depends what city you're in what the signals will be like, even if both transmitters are on Vashon Island. Have you tried rotating your radio to see if the AM710 signal improves, or changes? You might be in between two lobes of a directional nighttime pattern, whereas KOMO, I believe, runs a non-directional 50kw day and night.
Both KIRO and KOMO are non-directional daytime & directional at night.
Both stations' nighttime signals favor the northeast and southwest, with modestly deep nulls to the southeast and much broader nulls to the northwest.
KOMO is protecting Chicago -- their nulls point at that city.
KIRO's pattern is tilted somewhat more clockwise -- nulls more or less in the Denver direction -- although in theory they should be protecting New York.