There seems to be a lot of confusion about the line up of Tesoro coils. There are two basic families of machines and the coils are not interchangeable between them. On one side you have the newer High Output Technology (HOT) family of machines which include the Tejon, Vaquero, and Cibola and these coils also fit all of the Lobo series of machines. Coils that fit these machines can be interchanged with each other. Everything else Tesoro currently makes is called the uMax family of detectors, except the water machines. This includes the Cortes, the DeLeon, the Golden, the Silver, and many older models not currently built (Eldorado, Bandido machines, Sidewinder just to name a few). The uMax coils are not interchangeable with the HOT family of machines and Tesoro has put different connectors on these two families so it would be physically impossible for you to accidentally plug one in to the wrong machine.
Tossed into this soup are two types of coils: concentric and widescan (or double D). All of the current Tesoro machines are sold with concentric coils, that is the norm. Widescan coils, or double D coils, are generally better if you are hunting in mineralized conditions, otherwise they really dont buy you any more depth. A 575 widescan coil is no deeper than a 575 concentric under normal ground conditions. The detection area is a bit different with these two coils. Generally speaking, concentric coils magnetic field is cone-shaped with the deepest area in the center of the coil. Double D coils have basically the same depth but the pattern runs from the front of the coil down the middle to the back of coil, creating a straight-line pattern into the soil from front to back of the coil. You basically get the same depth at the front of the coil, as you do at the center of the coil, and at the back edge of coil. This style of coil makes it a bit tougher to pinpoint, and that is why you see guys X-ing a target left to right, then turning 90 degrees and doing to same thing to determine target center. There are some other techniques you can use but widescan coils can be a bit tougher to center with. Widescan coils tend to see little mineralization so where you tend to loose depth in mineralized soil with a concentric coil, you gain it back using a double D coil. They dont really give you any greater depth but instead stop you from loosing depth.
Tab-nabit, to answer your question, yes, you could put the 9 by 8 from your Cortes on the Silver and it would work just fine, but for optimum depth you might want to send the combination back to Tesoro to be sure they are balanced properly. Factory preset detectors (Cortes and Silver are factory preset machines) are balanced to the coils at the factory and if you change coils you may no longer have your machine/coil tuning at optimum. Will you loose depth? I would say yes but its really hard to say how much. It may only be a fraction of an inch, but its hard to tell. The coils could be tuned great or mismatched somewhat, but you wont know. Tesoro recommends you get your machine and coil balanced if you change from what was originally shipped with your machine from the factory and they will do this free of charge. If you have two coils you like using on your favorite machine, send them both in with the machine and they will balance the machine to best work with both of those coils. As performance goes, this is a minor loss of depth but if you are like me, you will want to be sure you are getting all the depth you can out of your machine/coil combinations. This coil and machine balancing is not necessary on machines that are manually ground balanced because the act of balancing your machine takes care of this issue. If you like having a arsenal of different coils, it would be best to get a manually balanced machine.
Larger coils give you better depth and coverage so you can cover more ground with each sweep. If you frequently have large areas to search, bigger coils do give you a slight advantage but there are some trade offs. Larger coils give you better depth but they tend to loose a bit of sensitivity to smaller targets. They also have a little more weight so you need to consider that factor. Smaller coils give you great separation for working in trashy areas and they are very light, but will lose some depth. The standard 9 by 8 coils on most Tesoro machines are the optimum coil to use in most conditions. Having other sized coils is a benefit and makes your machine more versatile, but its more important to pick the right coil for the right job. Its also a good idea, if you do buy secondary coils, that you also buy an extra lower stem for that coil. That way it is very fast and easy changing coils in the field.
Thanks for inputs 
