Frank, I have to really disagree with the above statement. The problem with this is the manufacture is typically making a product that works in many environment.. Not one that works really well in just one. Having different coils for different situations is not rocket science.. it is perfectly simple common sense.. I.E. Big area with little trash, requires a big coil to cover lots of ground. lots of trash within inches of each other, requires a very small coil to be able to pick up each target separately. After market products are required as the manufacturer may not want to design, tool and support multiple coils (actually a very complicated coil of many wires unlike your description) Take for instance Whites 8kHz machine.. they make one coil the 4x6 shooter, otherwise you are stuck with a 9" round.. The NEL hunter (close to the stock 9" size but DD rather than round) allows the machine to search a 10" wide (tip to toe) (DD coil) swath that is only an inch or so thick, at Depth for good separation . The 9" round only covers about 1" or so (bottom of the cone) at max depth requiring 75% or greater overlap on each swing.. I.E really slow progress to not miss anything) Also with the round you have a better change of masking with the 9" round field at the surface.. So tell me how buying aftermarket coils are bad? If you like to drive fast and make sharp turns and want you car to handle really well, you replace the factory tires which were chosen because they do everything "OK" with ones that are made for performance and handling under dry weather conditions.. Dont' expect to have good performance with those after market performance tires when it snows or rains..
Just like you don't use the big Tornado in a trash filled parks or the sniper to hunt the beach or corn field..
Ok Airscapes raise the shields, here it comes.
First off the technical info out there used to sell coils is pure BS.
The 9.5" stock concentric coil doesn't have a small cone pattern! I have pulled large targets at 2' with that concentric stock coil on a 14 year old XLT. I have found coins at 8" that read all the way across the coil.
So that blows the small cone theory.
I haven't seen a park loaded with junk in many years. They are hit to hard.
My XLT swings fast as I walk the ground. They are built for fast scan and can be calibrated for a fast reset. That is what I meant by get a better detector not another roll of wire to hang on it. If I hit two targets together, they both show on the graph. I then move in slowly from the sides to locate each. Why waste time playing with different coils?
I found this $3000 ring in an old back yard. It was next to a nail. The Graph showed iron and gold and the target ID shifted back and forth. By carefully moving in from the side, I located the ring and popped it out from app. 4" with my probe. That's dead on pinpointing of a multiple target with a stock 9.5" concentric.
You can believe that advertising BS if you choose, but I deal in reality. Sorry, but that's me. Frank...-
PS: Your car comparison is not realistic. Here's why.
If I want to detect in the park, a yard, or the woods etc. I use my XLT.
If I want to detect on the beach or prospect for gold, I use my Surfmaster PI .
If I want to cache hunt for a large cache, I use my Hays 2 Box.
No coil switching here.
? You shouldn't need a different MD to do a beach than the dirt! If one coil is good for everything so is one MD.. Like you also said in another thread.. All pin pointers are the same.. they pin point.. All MD are the same, they detect metal one should be all you would ever need.. All coils must be the same.. they are wire.. 
)
