"I've often heard that Levinson's amps have a "dark" sound to them. What does this mean?"
Here you go 333jeffery... An Audio Glossary
https://www.stereophile.com/content/sounds-audio-glossary-glossary
dark... A warm, mellow, excessively rich quality in reproduced sound. The audible effect of a frequency response which is clockwise-tilted across the entire range, so that output diminishes with increasing frequency. Compare "light."
Here you go 333jeffery... An Audio Glossary
https://www.stereophile.com/content/sounds-audio-glossary-glossary
dark... A warm, mellow, excessively rich quality in reproduced sound. The audible effect of a frequency response which is clockwise-tilted across the entire range, so that output diminishes with increasing frequency. Compare "light."