Grandma's technique probably had way more to do with it than the mixer itself. Creaming butter and sugar for a full five minutes by hand means you're watching and feeling every stage, and you stop at the right moment, but a stand mixer on high will happily turn that same butter into soup before you notice. Your grandma has also made that cake maybe 400 times, so her hands know things a paddle attachment never will. Give your stand mixer another year of pound cakes and I bet it catches up.