For sure the Sun Ray Pro Golds are a favorite among some detector brands. Not only do they have volume controls for both ears, but also have a limiter circuit so you can crank the volume all the way up and still not get blasted by surface targets. I've read that you can hear a click when the limiter kicks in so you are aware the volume of the target has been reduced so you can still judge how deep it might be.
One of these days I'm going to pick up a pair, because on my Sovereign Minelab says the volume should be cranked all the way up on the machine and then adjusted on your headphones to insure you hear the deepest of targets. So far my testing doesn't show that unless in pin point mode, which seems to add about 2 to 4 more inches of depth with volume at full versus lowest, while in discriminate mode I'm not seeing any difference, but am going to test that more to be sure. I seem to be able to hear the deepest of targets with it lowest, but for sure with it loudest you can more easily hear the "meat" or traits of the target at fringe depth for better decision making.
Right now I'm using some $20 Sonys that somebody tipped me off having really good audio and I must admit it's the best I've used so far. They are studio phones so half the size and super light compared to regular headphones, which I really like. I don't like my ears sweating and I don't like not being able to hear some noises around me to keep tabs on my surroundings, so these thus far are the best I've ever used in many respects, including the audio and them being very light and not bulky. The ear cups are probably about the diameter of a half dollar or so.