I have a customer who recently upgraded his Mark IV to a DKC-900, and wanted to add larger Bluetooth enabled speakers. We used Rockville TM150B 10″ Speakers. Although the speakers are powered, and amplify signal quite well, we nonetheless found the audio output from the Disklavier was a bit low. I added an RCA preamp to amplify the signal going into the speakers. The RCA preamp is zip tied in place using 24″ zip ties. These work wonderfully for strapping components to the beams.
The speakers underneath the piano are present, but are no longer powered or connected.