I haven't heard it. Flat frequency response and low noise/distortion/crosstalk/etc. means that, when paired with a similarly flat DAC, it should sound like the headphones and nothing else. Provided there's no clipping, obviously.
I will remind you that, though this isn't the Science forum, the thread is designed to be an objective discussion of the amp's capabilities. The amp itself was designed for objective performance, not subjective sound quality. Subjective impressions beyond measurable phenomena don't matter for this amp. Not for the people it's designed to please.
If you'd like to provide a reason why it wouldn't sound like nothing, when not driven to clipping or otherwise used in a way it isn't meant to be, please do.