I used to say "flat frequency response". While that it important, there is such a wide variations in FR of the recordings I listen to, I couldn't get a flat FR unless I eq'd each recording.
There are some others like:
1) Consistent sound between drivers. I had a speaker with poly mid/woofer and metal tweeter. The drivers did not blend together at all and sound like two different speakers.
2) Dimensionality. As one reviewer wrote, when a system reproduced things spatially, you know it is working very well. Or words to that effect.
3) Transient response. For one thing, when you limit a driver in the frequency domain, you limit it in the time domain as well. I'm guessing that's one reason why speakers put a bump at 100 hz. It's to try to add the bass that gets lost from dampened transient response. It's why SET amps are so popular. Again, IMHO.
There are some others like:
1) Consistent sound between drivers. I had a speaker with poly mid/woofer and metal tweeter. The drivers did not blend together at all and sound like two different speakers.
2) Dimensionality. As one reviewer wrote, when a system reproduced things spatially, you know it is working very well. Or words to that effect.
3) Transient response. For one thing, when you limit a driver in the frequency domain, you limit it in the time domain as well. I'm guessing that's one reason why speakers put a bump at 100 hz. It's to try to add the bass that gets lost from dampened transient response. It's why SET amps are so popular. Again, IMHO.