The gizmag review highlights one of the reasons why I avoid any kind of EQ:
As I went on to test the Zo on both expensive and inexpensive MP3 players, hi-fi systems and televisions the results were similarly positive. Not everything in this soundgarden was rosy though. Having found that different portable situations called for different contour profiles to be engaged, the fact that the Zo only remembered the last setting led to a few more contour searches than I would have liked. As well as the facility to store a few favorite profiles for easy recall, some sort of numerical indication of profile intensity in addition to the color coding would also have been welcome.
They also had a couple of other niggles:
I also found the battery charging via USB to be a little restrictive and the apparent lack of battery level indicator did lead to the odd occasion where the device simply cut out without warning. Aside from such minor irritations, the painfully easy to use device significantly improved the listening enjoyment through every audio source I tried, except one. I connected the Zo to the headphone jack of my Marshall guitar amplifier and noticed significant problems.
This is obviously an area for which the Zo was clearly not designed but I was curious as to what contouring would achieve in such a situation. As it was, the intentional signal distortion proved just too much for the personal subwoofer and although it fared slightly better with my HoneyTone personal guitar amplifier, any thoughts of playing with the EQ via the Zo were quickly abandoned and the unit returned to the audio equipment it was created to be used with.
http://www.gizmag.com/zo-subwoofer-review/15208/
Probably the biggest single objection I have to the ZO is that several reviewers claim that they cant see themselves listening to music without it now that they have become 'addicted' - thats some serious crack.