IMO, you're barking up the wrong tree using stereo speakers to amplify an instrument. I would buy a pair of Roland keyboard amps. I've been through this and two small amps gives much more spread and tonal weight than one larger amp. Much more of the feeling of playing a real piano. Also, they are much more subdued sounding than studio monitors. I would get 2 of the ones with the 10" woofers. $350 each.
https://www.sweetwater.com/c525--Roland--Keyboard_Amplifiers
https://www.sweetwater.com/c525--Roland--Keyboard_Amplifiers