You should also also warp the response of your slider so that it is not simply outputting frequency values in linear fashion. One way to do this is to define the fourth argument of the 'range' identifier to something other than 1. This should be adjusted to taste but something like 0.25 should give more useful behaviour:
form caption("Untitled") size(400, 300), colour(58, 110, 182), pluginid("def1")
hslider bounds(10,10,380,30), channel("cutoff"), range(50,20000,10000,0.25)
0dbfs = 1
asig vco2 1,100
kcf chnget "cutoff"
asig moogladder2 asig, a(kcf), 0.4