Kevin
I am not an expert on the C$, but from reading the manual and doing some testing, the threshold setting is a trigger point on targets. +4 is more excitable allowing targets to be acknolwdged, and -99 is less excitable ignoring targets as numbers decrease into the minus segment.
My thoughts on threshold setting are the closer that the settings are to +4, the faster the target processing information, and is designed for low to medium soil conditions. As soil conditions become more minerialized, the target processing information requires a greater length of time to id a target. The target processing time is increased as the threshold setting is decreased.
Every VLF detector on the market looses depth as minerization increases. The C$ address this problem in two ways, by threshold settings and by using the auto trac system. The best settings for using auto trac is sens at 4 to 5, and thres at -5 to -10. As soil minerializations change, one has to adjust sens and thresh to maintain max depth and target id. I think that this is the biggest challenge to using the C$, adjusting any one of its settings the wrong way will result in maximum frustration for the operator.
Sorry for the long post, but to try to answer your question. With a sens of 8 and a thresh of -10, will you loose depth if you increase thresh to -20 or lower? If soil conditions are high, you would most likley gain depth by decreasing thresh to a certian point. If soil conditions are low, you would loose depth if you decreased the thresh settings. The only way to find out which settings will give you the best results for your specific area, is to bury quarters at 2" intervials up to 12" and try different settings, but keep in mind that the C$ is sensitive to sweep speed. I have gained about 2" in my test garden just by changing my sweep speed from slow to moderate. The C$ is a strange bird, I like to think of it as the mocking bird of metal detectors. It acts like other detectors that I have used, but change a few settings and it acts like a whole different detector. Hope this helps, its only one opinion. TB