It seems odd to me that a manufacturer would post output specs at 10% THD. Who cares?
Why wouldn't they publish specs for a listenable THD?
The chips are not meant to be 'high end' FWIW. 100 watts is where the amp clips and at that point its making 10% distortion. So that is the spec they published.
but I heard that Class D amps lose a lot of their power (more than Class A or A/B amps do) when they drive speakers above 4 ohms, and my speaker happens to be 8 ohms. Is this so?
No. Most solid state amps will make less power into higher impedances. So you have to look at the 8 Ohm rating to see how much power the amp makes into that impedance. Driving 16 Ohms will further cut the power in half... you see how this works? Its not a 'class D' thing.
IMO if you are planning to use this for guitar, you'll be better off with a tube guitar amp of about 25 Watts. I think you'll find that it plays just as loud but is a lot more musical.