to get a near log performance from a linear pot you want to slug the wiper to the CCW terminal with around one sixth of the pots value. So if you have a 50K lin dual gang pot you want to use something like a 8K6 resistor. The only issue with doing this is that the impedance looking into the pot varies all over the shop depending on its position. In 90% of situations so long as the minimum impedance which occurs when the pot is fully CW isn't too low then this isn't a problem.
Your desire to make use of the 50K pot you have needs to be thought about. Is this a really good quality pot with good matching between the two gangs? If not, why bother, grab any cheap arse pot that is 10K log if you plan to implement the approach in the AT article, which is different to most where the pot is forming a variable shunt in a pad arrangement. But also give some thought to the concept of a passive monitor controller where there is little to be gained if it isn't better in performance than any other way of turning your speakers up and down. The Alps pots are getting cheaper ( here in Oz through RS components ), so in the scheme of things it is no big deal to use a good quality part that makes the whole effort worthwhile,
Your desire to make use of the 50K pot you have needs to be thought about. Is this a really good quality pot with good matching between the two gangs? If not, why bother, grab any cheap arse pot that is 10K log if you plan to implement the approach in the AT article, which is different to most where the pot is forming a variable shunt in a pad arrangement. But also give some thought to the concept of a passive monitor controller where there is little to be gained if it isn't better in performance than any other way of turning your speakers up and down. The Alps pots are getting cheaper ( here in Oz through RS components ), so in the scheme of things it is no big deal to use a good quality part that makes the whole effort worthwhile,