Cerwin Vega owner

The definition of good varies by individual experience.  If I were you, I'd stay right where you are and enjoy the music instead of looking for flaws and allowing it to become an obsession as so many audiophiles do and spend your $ elsewhere! It took me decades to reach audio contentment!

As with everything in life, the more/better you experience the more you'll want and the less content you'll become with what you have.  If you like it the way it is, stay with what you've got.  As the saying goes- If it ain't broke don't fix it! 

I would invest in a book and learn about acoustics and speaker positioning.  That will go a long way into improving your system and room at very little cost!

Engineer Paul Mcgowan of Ps Audio has written various books on audio that I'd highly recommend!  They're sold on the Ps Audio website and also Amazon.

  Happy journey...

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!

cleeds

What I can say is that with all things being equal, an amp with lower THD will have less hiss with NO SIGNAL when you put your ear next to a tweeter. The components inside of an amp are not silent.

1. • Hiss is one form of distortion. It often manifests as a faint, high-frequency noise in the background.
   • High THD can contribute to increased hiss because it distorts the audio signal, including any inherent noise. As THD decreases, the overall noise level also decreases.

cleeds

The point is that you said that a signal is necessary to hear THD. That is not the case since there is no need for a signal going into the amp to hear hiss, and not all amps produce AUDIBLE hiss or noise on their own. My better ones are dead silent as far as hiss or any noise is concerned even when touching my ear on the tweeters.

The OP asked what THD sounded like and I replied with my suggestion on how to hear it but you instead challenged my post and then when I clarified it for you, you suddenly didn't make sense of it.   Good day...