From the manual:
"The rear ACOUSTIC PRESENCE control adds a sibilance to the high frequencies of your guitar.
Most presence controls work in the 3k to 4k range. However, the VINTAGE TUBE’s presence starts
at a very high 8k Hz delivering 10 dB at 12k Hz and continues to 20k Hz which extends all the upper
harmonics of your guitar. The amount of sibilance will depend on the speakers used. To keep both
channels totally independent, the ACOUSTIC PRESENCE is switched by relay only into clean channel
1. The effect of the ACOUSTIC PRESENCE will seem ever so slight, however, the result is added
sibilance only to the ultra-high frequencies."
I suspect it's working and, as the description says, ever so slight. If you're running a typical guitar speaker, frequency response may not even include 8K or higher and you would hardly notice it at all since we're in tweeter range.