My wife's digital piano has a couple of keys that output distorted audio. Unfortunately, it's a Technics piano, and they're no longer supported. So, I'm doing some debugging. Walk with me through my hypotheses and see if you agree with my reasoning...

Initially, we suspected that it was something to do with the key switches, because it's only a couple of the keys that are defective.

The piano has various voices "Grand Piano, Upright Piano, E. Piano, Strings, Vox, etc.". If you switch to a different piano, the distortion remains, but if you switch to "Strings", etc. the distortion goes away. This suggested to me that it might be the waveform ROM that had been corrupted (on the assumption that all of the piano modes used the same waveforms but meddled with it).

However, with headphones, you can't hear any distortion, which suggests that it's actually the speakers that are at fault. But why, then, with only a couple of keys, and only with certain piano modes?

Further: in an attempt to work around this, and rule out the built-in speakers, I'm considering running the line-out to a separate amp and speakers, but I can't be bothered to move (or run cables to) my existing audio receiver. What search terms should I be using for a small basic amp that has line-in and twist connector output?

The piano's a Technics SX-PX554, incidentally.
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-- roger