That's right. The empeg probably puts out a bit less than 4V at full volume. Some head units put out close to 9V. I have no idea what the Sony above is rated at, nor if it was using line-level or speaker-level connections to the amp (which I also don't have details on )

At home (where the empeg is rated at 1V), set to 0db, mine is not as loud as my Denon CD player connected to the same Denon amplifier.

In the car I have far more control and can make sure it's at a level very close to the CD player built into my Clarion HU.

Bruno