Welcome to the forum!
It shouldn't hurt to try going directly into the power amp. Turn the input levels down on the amp, then gradually bring them up while the keyboard is playing.
Most everybody uses some sort of mixer, however, when using a power amp. There are many good, inexpensive choices available. Alesis and Behringer both make small mixers for $100. or less. They would give you more control and more options.
DonM