digiZoid ZO2 Listening
The first thing I was worried about with the ZO2 was that it would affect non-bass frequencies in some way. I did extensive listening tests with the ZO2 and I could find no evidence that it affected any frequencies other than bass in any way. If I was listening to music that had no bass (songs with a cappella openings for example), I could push the bass boost all the way into red without hearing any sort of change to the presentation.
With that, I focused on the bass end. Regardless of the name, the ZO2 Personal (not a) Subwoofer will not create bass where none exists. It can't cause a driver to recreate frequencies that are outside of its range. What it will do is give the driver more power (amplification) to work with and accentuate (bass boost) those lower frequencies.
The bass boost was smooth and lifelike. Unlike some of the digital solutions I've experienced, the ZO2 boosts bass in such a way that it truly sounds like it was recorded that way. With bass heavy headphones like the Moshi Vortex Pro, I preferred little or no bass boost. But with bass light ones like the Pioneer SE-MJ591s, I really fell in love with my headphones all over again. This is what the headphone lacked and the ZO2 has such a natural and lifelike presentation that I couldn't believe that it was an external force creating the extra bass.
But if you really want to know where the digiZoid ZO2 shone, it was with the Arctic P402s.
Yes, I know that pairing a $120 accessory with a $40 set of cans is idiotic. I admit that taking the $160 you'd have spent on the two and getting a better set of cans would be a much better investment. But I think of it this way - there are tons of high-end headphones out there that need external amplification because they are so hard to drive. While tube headphone amps may be pretty, they aren't exactly transportable. So, rather than focus on the cost of the Arctics, I'm going to focus on how hard they were to drive.
And they are hard to drive.
If any of my other headphones required 1/3rd volume to reach a comfortable listening level, the Artics required 2/3rds or more. Easily double. With the amplification inherent in the ZO2, the volume increase was nullified. This takes a huge load off the source device (usually my phone, iDevice, or computer) and also provides much more headroom. The increase in sound quality was immediately noticeable. It was clear that even at moderate volumes, the amps within my source devices were near or at clipping with the Arctics. With the ZO2 in place, the presentation was much less strained and constricted. Given that the Arctics weren't exactly the most bass heavy of headphones, the digiZoid ZO2 really made a huge difference. So much so, that from the midrange down, the Arctics sounded like headphones two or three times their price. The high end gave them away, of course, but the difference was pretty amazing.
So I can get this one for a whole lot less and use my EQ settings to boost the bass:
FiiO E11 Headphone Amplifier 230-106 [parts-express.com]