processing analog synth, deriving different filter types

so i saw the tip on obtaining a highpass filter output from a lowpass filter in the "did you know" column. you know, the one where you subtract the original signal from the lowpass filtered signal as a means of obtaining a highpass filtered signal. got me thinking, any fancy math that i could utilize to perform a similar function with external gear into max? say, my raw oscillator out and 24db lowpass filtered osc out into msp? or maybe just some other tips and tricks along those lines that some of you know of.