I agree with Hombre and Barber Bill.
If the ground you are mainly detecting is heavily mineralised, or varies from moderate to heavily mineralised, then you would be better served, as Barber Bill suggested, looking for a detector which has variable ground balance control, so that you can change the ground balance to suit the ground conditions.
Merely buying a DD coil, and a very large one at that, probably wont help your detector, which has fixed ground balance (pre set at the factory to try to accommodate a variety of soil types to some degree).
DD coils can be better suited to some ground conditions than concentric coils, and concentric coils can be better suited to some conditions than DD coils. Sometimes it can be a matter of trial and error, testing different coils in YOUR ground, to find the type that is best suited to YOUR ground and conditions.
In some conditions, using a large coil wont improve detection depth over a smaller coil. If there is a detecting club near where you live, you should be able to find out from the various members which types of machines and coils work well in your conditions, and for what purposes (ie, coin & jewellery hunting or relic hunting or gold prospecting).
There are a multitude of detectors, and even more coils out there, and finding the right ones and the right combinations for you can take a long time, but the main thing is to have fun while you are finding out. HH