Just like the rotary "Cut" control on an AC-30, the "Dark" switch combined out of phase high frequency signals coming from each side of the phase inverter. This caused a cancellation of treble frequencies when the "Dark" switch was engaged.
The Vox/Bruno utilized a single 12 inch, 16 ohm Celestion speaker.
The full rear panel of the amplifier cabinet interracted with the speaker to increase bass response. Three narrow horizontal slots were cut into the back panel. These slots would limit the the bass cancellation one would expect from an traditional open backed amplifier cabinet. When combined with the 6V6 output stage, this semi-closed back cabinet design gave the TB18C1 a significant boost in low frequency output. The slots also provided a source of cooling air for the chassis.