Hi Martin,
There are multiple factors that can influence which earphones would work best for you. First is your hearing. If your hearing is great, then many of the less expensive may work. If your hearing is poor like mine, then you will need to be more careful in your selection and probably have to pick a set with a higher impedance for the added volume.
Also, there are distinct differences in the audio circuits of different metal detectors. On some, a person can use 8 ohm earphnes and have a loud response. On other detectors, one can barely hear a signal with the same earphones. So, in a nutshell, one really should have an idea as to the output of the detector.
I have never heard anything bad about Gray Ghost mentioned earlier. On the contrary, everything I have heard is positive, but I personally haven't tried them. I have used different Koss earphones including the TD 81, some of the earlier models, and also R 80's. All have worked quite well.
I have also done something like Randy mentioned. That is to buy a cheap pair of earphones like the basic Calrads that have volume controls and a coiled cord and then installed different speakers. I have used relatively cheap 90 ohm speakers I got from All Electronics for about $1.50 each that worked great. This increased the volume greatly.
There are exceptions, but as a general rule, the higher impedance of the earphones, the greater the output volume you will have. The type of cord is important under some conditions. If you are going to work in brushy areas, then coiled cords work much better than straight cords. Volume controls are also an asset in most cases, especially if different people are going to use them or you area going to use the same earphones on different detectors.
Finally, depending upon the machine, some people prefer to buy a preamp to use on their earphones. I have heard good reports on one of the Koss models and their use on Minelab instruments.
Sorry, but the answer to your question is really not that simple.
