Rob N,
My summary of the listening review of the EarMaxPro (EMP) conducted by Muse Kastanovich follows. I will refer you to the original article for exact details as many of the terms used in describing the sound are often subtely shaded and my comments may unintentionally give the wrong impression. I have quoted where needed to attempt to avoid distorting the results.
Sony MDR-CD999: With the EarMaxPro, these phones had a "mellow sound that gave voices and drums a nice genuine feel, but guitars and rasp were a little too soft. This particular combination had a wonderful presence and lively dynamics that kept the music very involving." The high-end of the frequency response was slightly attenuated, although the author's use of Silver Streak interconnects was reported to help this somewhat.
Grado SR80: Very detailed and well porportioned. The treble was slightly "too hot" when using the Silver Steaks, so the author recommended the use of copper interconnects, such as the Kimber PBJs that he used. "Stunningly good, almost a match made in heaven."
Grado SR225: Mostly general comments about these phones compared to the others. Reportedly "far more detailed and refined" than the Sonys. Similar in sound to the SR80s, but with smoother bass, as well as more precision, detail, and dynamics.
Although the Sonys were reported to have better frequency balance then the Grados, the Grados were reported to be better in every other area and, as a result, the rest of the tests were conducted solely with the Grados.
The EMP was then compared to several other amplifiers: Amphony model 1000 headphones with integrated head amp; NAD 1300 to provide a reference to a receiver or preamp; and Parasound Zone AMPlifier (or ZAMP).
Amphony: Author states that the ZAMP amplifier sounded better than you would think when judging it by its low $129 price. Still, the Earmax was superior in transparency, spaciousness, pace, rhythm, smoothness, and timber, and fidelity.
NAD: EarMax reported to better separate different instruments and provided a superior richness of sound. NAD mushed the instruments together and had overall graininess and thinness. The EMP was superior in every quality except for "perhaps the top octave, where the NAD had just a touch more of the pleasing shimmer."
Parasound: Dynamics were good, but not quite up to the standard set by the EMP. Frequency balance was reported to be nearly the same as the EMP. The Parasound had better bass response than the NAD and less graininess in the treble. But the NAD couldn't match the EMP in smoothness or transparency.
EarMaxPro summary: "overall balanced sound" with a "natural realism that allowed listening right down into the music." Though expensive, it is said to be "extraordinary sounding"
After this review, I am sorry I have not had the opportunity to hear this amp yet. I hope this information is useful to you.