I have the PX100 and I'll say its good but its definately not flat response. In fact its mids are rather scooped which is kinda good for metal and hard rock stuff.
What do you plan to use it with? With a good mp3 player most earphones can be eq to get flat response. Otherwise, there is no flat response earphones unless you get up to 3-driver models.
Going to have to do demos.
Been using my sony noise canceling plugs.
Had read an old article http://gilmore2.chem.northwestern.edu/articles/hoff_art.htm
about them not having flat response but its been 10 years.
Guess it will be mufflers.
Found a cheap model AT M30 at $66.