Trace Elliot 1215 HF Horn problem

[Guest MoJo]

My recently acquired 1215 combo has a selector switch on the back to control the HF horn. Options are Low-Off-High. Moving the switch to any position doesn't really make any noticable difference to the sound. If I push the sliders up on the high end of the EQ and wind the high band compressor up, I can hear a hissing from the horn, so it's doing something (but not a lot). Sharp contrast to my old Peavey 410TVX cab who's horn could shatter glass if wound fully up.

Is this how they are or is this one broken?

Cheers