Electronics voltage question?

If I wanted to increase the power of a metal detector AND increase the size of the field (to go deeper)

It already does a good job of controlling the sensitivity and generally functions to only detect worthwhile volume. Anyways. Most important question:

If I increase the voltage from 12v to 18v (I have 4 rechargeable NiCD that work good I can carry in my back pack up high to avoid the field). I want them still available to my drill and saw so no modification to the battery. I've already done that with one, because it had 2 bad cells anyways).

But I figure 18v would perform better than 12v in terms of creating a field right? Which components need to change? I know the caps have ratings printed on them. Most say 15V So I would change them to something higher than 18v

I assume that resisters should be fine except I would have to compensate for the intended current draw by viewing the schematic and calculating the resistance figures, to establish amperage against voltage, then convert with new voltage and desired amperage (It's a thick board with heavy lines. It should handle 2 amps max no problem. I'm willing to recreate the board entirely from scratch. The batteries are only 2.4Ah but I would consider a cart and car batteries if I could achieve 10 feet depth with a 2-box detector.

I'd love to figure out the pot range to make it adjustable. Which is probably what the sensitivity knob is. The dots may not work anymore if I change it too drastically. (they help you calibrate at start up)

So is there anything else that would need to change for a 6v voltage increase? Diodes? Microchips?

Any good website recommendations where I can easily reference information about electronic components that would be awesome. You know, like specific specs and schematics for chips, etc.

Any and all helpful information would be very much appreciated.