How to subtract filter responses from one another?

Hello.

I have simple question: I have two filter magnitude responses with the same sampling frequency and range. I'd like to see the difference of the two responses to visualize the benefit of using the second filter (i.e. at which frequencies it behaves differently in terms of gain).

I've used fvtool(a,b), where a and b denote the two input waveforms to do this, I just would like a confirmation that this command is what gives the above mentioned 'subtraction'.

Thank you.

Tamás