I have a selection of coils built up over the last six years.   Most are DD or Anti Interference,  as I hunt in the UK for small hammered silver coins I often need to be able to discriminate iron.  I also have a small selection of monos'.
My questions are:
Will a fully modified GPX using either Coiltek/Minelab DDs or Coiltek AI coils give extra noticeable performance?   I guess the Coiltek/Minelabe mono coils will benefit but I am not sure about the DD & AI.
My biggest problem is not ground noise but EMI from suburbia/power lines/mobile phone transmitters etc. Sometimes, I cannot get a completely smooth threshold.  Will a full modification enable smooth threshold most times?
Is there an optimum coil size for a modified GPX?   I use both the 4500 & 5000.