Hey Tony, I am by no means an expert on tuning coils to a detector, but I have several coils for each of my detectors and never sent one it to be "retuned". The reason is because I swap coils a lot, using different coils for different applications. (isn't that why they sell extra lower rods?) And, if they have to tune the detector for one specific coil, then obviously it will be "out of tune" for the others. In other words, if they have to change the tuning to accommodate your 5.75, then it would be off for the stock coil.
From a personal standpoint, I really like the 5.75 HOT coil for the T/V/C. Once I got mine, I seldom use the stock coil.
Kind of funny you posted this today. I also have a DeLeon with the stock coil, the 4-inch coil and a Troy Super 7 coil. (by the way the Super 7 is my favorite Umax coil). Anyway, I recently bought a new 7-inch widescan Umax coil and it wouldn't stabilize. Chattered all the time, regardless of soil conditions or settings. Wouldn't work on my DeLeon, a Cortez or a buddy's DeLeon. But, my other 3 coils work fine on all three detectors. So, I am sure it is just a bad coil (or the widescans are poorly designed). Anyway, the dealer I got it from bailed out on me, so I wrote Tesoro and asked them how to send it in for repair. They told me I had to send in the detector control box along with it so they could "recalibrate" the detector for the widescan coil. I told them I was confident that the coil was the problem and felt no need to send in the detector. I told them that I didn't want the detector recalibrated for the DD coil when I planned to use the concentrics 90% of the time. They reiterated that they were unsure they could help me if I didn't send in both the coil and the housing. Well, I mailed in the coil today. And I did not mail in any of the three control boxes that the coil wouldn't work on. I did include a note that instructed them to throw it away if they couldn't fix it and wouldn't replace it. No use to waste $ on shipping me back a coil that won't work. It will be interesting to see if they fix it, replace it or just simply throw the coil (and my future business) away. I'll keep you posted. HH Randy