The advantage to running with ID norm off is that all the ID's of the lower operating frequencies are NOT crammed at the higher TID's associated with 18 khz, in this case if you are running at 25 khz, then they are down averaged to 18 khz which is actually an advantage when running the highest frequency on the X35 coil. One other advantage of running with ID norm, however is that you can use the fact that ferrous trash like bottlecaps tend to misbehave TID wise and they do not follow the convention that TID increases with operating frequency (with ID Norm off). For example say you were trying to differentiate between a bottlecap and quarter that were both ringing up in the low 90's. By lowering the frequency to say 4 khz, you would expect the quarter's TID to lower into the 80's. A ferrous bottlecap or other round ferrous junk ID will actually rise or get jumpy when you do the frequency shift. You cannot use this trick when you have ID norm on because all the TID's are the same regardless of frequency. The downside of not using ID norm is that you have to set different tone breaks depending on your operating frequency.
Regarding your program, if I am running multi-tones or pitch, I dial in discrimination up to 10. It has no effect on depth, prevents ferrous down averaging of TIDs, allows the horseshoe depth display to work properly gives you an extra tone, and I can still hear the iron using iron volume. The only time I run without discrimination is when I am using full tones (because iron volume does not work in full tones, so if you dial in any discrimination you will not hear the iron). There really is no upside to running negative discrimination in a multi-tone setup.
