Svart,
IIRC, the gain for a 3-pole Butterworth equal value Sallen-Key is 2.0, but you have to put a buffer after the first pole pair. That isn't much gain, so overload isn't likely to be a big problem. Usually you loose some amplitude when you highpass filter audio signals because most of the power is at lower frequencies. You can always pad it down by 1/2 before the filter if you need to.
In other words, if you want to use a 3-deck pot for your R's, the caps can all be the same value. The circuit goes like this
RC HPF -> op amp buffer -> 2-pole Sallen & Key HPF with gain of 2 on the op amp (two equal value resistors in the feedback divider).
Putting the gain stage second reduces the chances of overload if your signal levels are pretty hot.
If it's gotta be variable, and you want to turn a knob and keep a textbook Butterworth response, this is the only way I can think of other than the 3 -buffer approach bcarso suggested... except some gosh-awful multideck rotary switch nightmare <grin> The different R values and the loading of the first RC section by the second keeps the topology you posted from working well in a variable filter.
Don't forget to put equal value fixed resistors in series with each pot section so your filter has a maximum corner frequency stop point. If you let the pots go to zero ohms it may not be pretty. :shock:
Using 0.022 uf caps, that makes equal R's of 14.4K at 500 hz, and down to around 12K at 600 hz. if I calculated it right. So, like three 5K pot sections and three 10K fixed resistors in series with each section gives you a little wider range than 500-600 hz.
Now all you gotta do is find a 3-deck 5K pot :roll:
Hope this is what you're looking for.