I spoke with Alan a bit on this one also and we concur that the amplifier is being adversely affected by the musical instrument. We would also caution to keep the volume to a reasonable level so as not to damage the M3s but I doubt that will be an issue in a living room environment. Many of these instruments generate sustained frequencies, ultrasonic harmonics, and intermodulation distortion (IM) that may damage tweeters or woofers when fed directly to hi-fi speakers not intended for musical instrument reproduction. Using a separate wide bandwidth high-fidelity amplifier only exacerbates the problem because, unlike musical instrument amps, their bandwidth remains linear to 60 kHz and beyond.
Alan thinks the problem may be that the keyboard is overloading the input circuit of your Audio Refinement amplifier--especially since you switched amps and the problem became more prominent. The reason you do not hear the distortion over his Sennheiser headphones is they are plugging directly into the digital keyboard, and the keyboard's internal headphone amp is designed to filter out ultrasonic distortion and harmonics.
