I have a pair of the GG amphibians and I think MikeK above has the description of the sound level about right. I think the cheap Minelab ones (not waterproof) that come w the CTX 3030 are loud (and fit poorly - too big even for a BIG HEAD). But they are loud. I also have a pair of the LS Pelso waterproof Yellow headphones from Kellyco and for waterproof headphones they're louder than the GGs.
Had to return the GGs after about 4 months surf use because of loss signal (plug/wire problem). Got them back and never used them and just today noticed that the male plug for the GGs is bent and distorted and won't plug into CTX. Aaaauuuu. [Don't know if they were damaged when repaired, but damage could have occurred by having plug in a vice, for example.]
Had the LS Lesco headphones for about 2 weeks before they went bad (same problem as CTX). Were quickly replaced by Kellyco.
Have been testing various waterproof earbuds for my detectors (Bluetooth for Equinox 800) and for the CTX found a nice wired pair of earbuds to use that have a waterproof rating of IPX7; Ok if dropped in 3 ft of water. Would have to add a waterproof connector for the earbuds to connect to the CTX for water use or one could use the WM08 module w the earbuds and go detector wireless. I've been summer land hunting in 90-100 deg heat and earbuds are the way to go. The only problem in using the WM08 module (or Bluetooth on 800) is that you can't place the control unit underwater because of loss of signal, effectively limiting surf hunting to about 3 ft or so.