not modified or even if it is able to interchange search coils, you have to remember that it is a fixed, or preset Ground Balance model. With Tesoro units, it is important to remember that Ground Balance is very important, and needs to be able to handle very mineralized conditions w/o falsing, but also not have a too-positive Ground Balance setting. That could cause the loss of responsiveness to larger silver coins, such as half dollars or big silver dollars.
Another point to remember with many [size=small](most ?)[/size] metal detectors, and especially with most Tesoro models, is that there can be quite a different in the Ground Balance setting or reference point between search coils. That's why it is suggested that if you have two or more coils and you experience falsing or erratic behavior ... with a factory preset model, ... you can send the detector and coils to Tesoro to be 'calibrated.' What is done is all of the coils are checked to make sure they are "in spec" and not defective, then they use all of the coils to find a GB setting that can be used so all will have ample GB adjustment so as not to false. They might not be a perfect GB setting for all of the coils, but it will be workable.
As for the Compadre or any other model that relies on a preset GB and isn't therefore as versatile as a manually GB'ed unit, my suggestion is to select the search coil you feel will be used the most for the types of sites you hunt. For ME, on MY preset GB Silver Sabre µMAX, I have mounted a 6" Concentric coil [size=small](measured, but Tesoro calls it a 5.75)[/size] because it is light weight and balances well, and provides ample performance for the sites I'd likely grab that detector. That coil stays on that detector, just like a similar Concentric coil stays on my Bandido II µMAX.
So pick your coil, but from the various Compadres I have used and compared, I'd go with one that has a 7" Concentric or a 2nd choice would be a 6" Concentric.
Monte