Probably because the KenRad was being used as the driver. It will add a little more than bass to the Sylvanias. This is a nice amp, but don't use it to drive current hungry headphones. Be careful in that regard. Stick with something that is relatively high impedance. At the time these were made the Senn 650 was the headphone of choice, but anything with a rated impedance of around 100 ohms or more will probably be fine.