But lets start with a correction. Minelab FBS does NOT use 28 frequencies. ITs been shown repeatedly that they use 2 frequencies and everything else are natural harmonics off those frequencies with logarithmic drops in power the farther they get from the transmitted frequency.
No changes you make will significantly alter the frequencies used. Those are set. The only changes to them comes from the noise cancel channel selection which shifts the frequencies a tiny bit up or down but nothing major.
Last, if you block out (discriminate) certain ares of the display, it makes no change to power, frequency, or anything else. The detector brain simply looks at the target return FECO and if its allowed, makes the assigned noise, if its disallowed it block the noise.
Like Jason said. What counts is not how many frequencies are transmitted, but how many of them are actually analyzed by the metal detector's receiver section.