Group 1: To ME, these are the budget-priced, low-end headphones, or the 'average' headphones they often advertise as "made for metal detectors" and usually run in the $29.95 to $49.95 price range. Some have a thin straight cable and those tend to get kneeled on or otherwise snag on things. Some are also too short. There are lower-cost models with a coiled cable that is very short and doesn't stretch much, at least not enough to be practical, and those tend to tug at the headphones and yank them off.
Many of the 'budget-priced' headphones are too light and don't cover the ears enough to block outside noise, Some do have somewhat larger ear cups instead of the tiny size, but they often fit too snuggly and press on the ears, and that's uncomfortable, plus even though they press on the ears they are also floppy in that they can fall; off if you move your head too much or too fast or flip it forward or backward. Also, most use speakers that are maybe 8 Ohm or possibly 16 Ohm, but generally that or 32 Ohm impedance. To many people, once they hear the audio of a better-quality headphone, most of these lower-impedance, lower-priced headphones sound kind of 'muffled' and don't provide a nice, brilliant audio response.
You want to use a quality-produced metal detector and hear a pleasant, quality sound to help find good targets, so I would pass on anything that falls into this category.
Group 2: High-quality headphones with a comfortable fit, snug enough to stay on and not flop or easily get tugged off, with a functional length coiled cable, and speaker impedance in the 50 kHz to 150 kHz range. This group will have a similar-size ear cup that is what might be considered 'normal' or 'standard' for top-end headphones in the MD industry. Examples of these would be Killer B 'Wasp' [size=small](my personal favorite in this design)[/size] or Killer B and Killer B II; SunRay Pro Gold [size=small](made for them by the Killer B folks and similar fit and performance as the 'Wasp')[/size]; Detector Pro; and others. The majority of these use 150 Ohm speaker elements.
Group 3: High-quality headphones with a comfortable fit, and very similar in overall design as those in Group 2, except they use was is referred to as the 'tank' style ear cup. This is a fuller-size ear cup that more completely covers the ears and, as already mentioned, can be more comfortable for those with larger-size ears, or who use hearing aides, and they do an excellent job of blocking outside noises. A couple of examples of these would be Killer B 'Hornet' w/150 Ohm speakers, and White's Pro-Star w/50 Ohm speakers. There are a few others out there with the 'tank' style ear cup.
My primary-use headphones are the White's Pro Star mainly due to them being more comfortable with less pressure on the ears than those models in Group 2. They have a Lifetime Warranty, as do the Killer B models. I do travel with two sets of headphones, the Pro Star and my former #1's the Killer B 'Wasp.' I have a brand new set of camo Killer B II's at home in case one of my others get broken or otherwise fail, and I also have a new set of Pro Star headphones as well. Pick a quality built headphone from Group 2 or Group 3 and you will enjoy much better audio quality and comfort than with any that fit in Group 1.
Monte