I have now figured out that its the Headphones that determines how loud you can play music through a portable CD player, not the CD player itself. Now the question I have for you is, what 'thing' I guess you could say determines how loud your Headphones will be?
the "hertz" is frequency response (how many times the speaker moves for one second. human hearing range is about 20hz to 20,000hz. Your headphones should be that range or better.
wattage is a unit of power and that will be the "loudness", you might also see a measurment of decibels, that is sound volume also. These units are actually controlled 100% by your cd player (because that's where the power comes from). Headphones are doing nothing more but tunring the cd players output into sound. And, at least in the world i live in, any electromechanical conversion will result in some energy loss... that is why "efficiency" will be rated as well. 100% efficiency will be the best, but of course, nothing will ever be 100%. the closer the better...