the gains are mostly log scaled from 0.1 to 10, so that a value of 0.5 on the slider corresponds to a gain of 1.
the reason i used 0.49 instead of 0.5, is that i didn't like the appearance of the GUI sliders when it was exactly 0.5 (the slider line goes thicker)
a non-resonant lowpass only cuts frequencies without boosting anything, and the DIY lowpass uses 4 non-resonant lop~ objects in series. Pure bass tones shouldn't be affected too much, but higher frequencies will suffer some loss even with the cutoff set at max.
If you're making a kaoss pad style effect, then maybe you could do a hack to reduce the effect 'amount' when the cutoff is set very high. So, for example, when the cutoff slider goes above 0.75, the 'amount' is automatically reduced from 1 to 0 over the remaining range.