OK, since I still have mine, I hooked it back up. I don't have any fancy audio analysis equipment, but I do have the world's cheapest oscilloscope, and two very aged ears. Since the distortion was so obvious in the review, I figured I could have ago. And I did find something!
With one output enabled:
With two outputs enabled:
Oh dear! And with two enabled, the tone is very obviously distorted.
So... maybe it's just not designed to have more than one output enabled? The push buttons don't move much when they're on, so it's very easy to have one turned on and not realise it.
Of course, if you're not supposed to have more than one enabled, then why are they buttons and not another dial. That, I don't know.