That's pretty crazy. I notice the bottom end thinning when going above 8 too. I just marked the knob to point at me when at 8 with a marker in case I want to roll back up quickly.

Hi,

I have a Variax 500 as well and notice the same thing.

Here is my explanation:

from 5 to 10, the tone control of the Variax mimics a "variable cable capacitance".

It shifts down the virtual "resonant frequency" of your virtual pickups, making them beefier sounding, as if you were adding a length of cable (and the related capacitance) between a guitar with passive pickups and the input in which you plug it.

Explanations about resonant peaks of guitar pickups, if necessary (scroll down in order to see the graphs showing how capacitance changes the tone):

http://buildyourguitar.com/resources/lemme/

A cable is a kind of giant capacitor. Typically, each foot of cable adds 50picofarad to the circuit and lowers the resonant frequency of the pickup.

When the cable is long enough (around 6m / 1000 picofarad), the resonant peak of the pickup falls in the range enhanced by most guitar loudspeakers: the sound becomes creamier and apparently louder. IOW, the cable reacts as a passive mid enhancer...

Here is another link about it:

http://www.ovnilab.com/articles/cables.shtml

The tone pot of the Variax mimics this effect, while a regular tone pot with a "real" pickup does nothing else than flattening the resonant peak from 10 to approximatively 3/10. You can see it here:

http://www.seymourduncan.com/blog/the-tone-garage/stop-ignoring-those-knobs/

FWIW: my answer is based on hundreds of actual tests with "real" magnetic pickups and on extensive experiments with my Variax (IOW: it's not a supposition; it's something sure IME).

HTH,

FF