As far as THD, it will be most noticeable whenever your volume is turned up and the source is muted. It will be a hiss.
I would never use anything less than 14-gauge speaker wire, depending on distance and wattage. I am not sure how to describe the difference in sound with words between different gauges of wire, but you will hear it with good speakers! 16 gauge is way too thin. I like to use oxygen free finely stranded thinned wire of at least 12 or 10 gauge. The material, construction, manner of extrusion and other variables of the wire has a lot to do with it also.
Specs are a good reference point but should not be the deciding factor when you buy something but a THD of .9% is very high and undesirable though. To me the musicality of any audio component is the most important aspect. People can give you suggestions, but everyone's system, room and ears will be different. Musicality is something that only you can judge in person in your specific room, ears and with your equipment.
This link gives you some basic info, but there are many other specs such as damping factor, slew rate, amplifier class etc. to consider that you can google. YouTube is also a good source. Understanding Amplifier Power Output Specifications (lifewire.com)
If there is a specific question that you have feel free to send me a message. I'll do my best to answer for you!
