I don't think changing the temperature of the Grand to either hotter or colder, will do any good. The atty base is probably free machining brass and the connector is probably 303 stainless. If so, then the coefficient of expansion of both metals is near the same, which means heating or freezing the Grand will expand or contract the metals at the same rate.
If the needlenose pliers don't work, then you could find someone that has some LN (liquid nitrogen) in a spray can, like a doctors office, then they can spray the atty base with it and shrink it. Then it should be easily to unscrew.
If that fails, then you can get a small easy-out tool from a hardware store that will fit in the center hole of the atty base and lightly tap the easy-out into the center hole to engage the easy-out into the brass. Then turn the easy-out counter clockwise with pliers or a small crescent wrench and the atty base will unscrew. I think they don't make easy-outs that small, so you may have to carefully drill the center hole larger to fit the easy-out. Be careful not to go too deep and drill the connector.
if you don't want to mess with it, then send the Grand to Rob. I'm sure this is not something new to him.