I bought the Headquake app for my iPhone after listening to an interview with the apps creator on the "Home Theater Geek" podcast (CEO or something-or-other of Sonic Emotion). He sounded like the real deal, so I sprung for the app (a whopping 99 cents on the iTunes app store). I've now listened to music with it for several hours, so I feel like I can write a competent review. I'll preface by saying the following are all subjective impressions. I make no claims as to objectivity.
[Listening done using an iPhone 4, no external DAC, the amplifier section of a NuForce Icon HDP, and three sets of phones: Sennheiser HD 570, Shure SRH 940, and Kliptsch S4i.]
The good: this app's digital processing makes some (but not all) tracks take on a sense of width and depth that the studio master didn't otherwise impart . I found it particularly enjoyable with many live tracks. There is a button to disable all processing so you can A/B with and without the effect. While many audiophiles hold up the studio master as the paragon that all playback equipment should strive to replicate, we all know that some recordings are crap, lifeless. This app has the potential to breathe life into lifeless recordings and create a very convincing soundstage. The effect was more convincing when listened to using the two sets of circumaural phones (Sennheiser HD 570 and Shure SRH 940), than when using Kliptsch S4i in-ears phones. I don't know if one can generalize that the app works better with over-ears than in-ears, or if there is something about the in-ear filter in the app that just doesn't work well with the Kliptch. Finally, the app has two slider bars that allow you to customize the effect to suit your tastes, although I found the default effect to usually be the most convincing.
The bad: First and most importantly, it's fatiguing on my ears. For that reason alone, I don't use the app for serious listening sessions. Second, it imparts on some (but not all) tracks a hiss that I find annoying. This is a hiss that isn't otherwise audible without the digital processing. Third, sometimes drums lose their crisp attack, like I'm listening to them being played in a cavernous space from a distance away and their sound is getting swallowed up. Lastly, and this should be obvious, any improvements in soundstage come at the expense of transparency. Make no mistake, this app is imparting its stamp on the music you are listening to.
Addendum: If you read the reviews on the app store, you'll find several people complaining about not being able to multitask with their phone while running the app in the background. This is true -- the app becomes spotty when it's run in the background -- but I do not fault the creators for this. This app is doing some serious digital processing, much more than simple EQ. It deserves to be in the foreground while it is running. I don't have access to an iPhone 4S or iPod Touch 4, so I can't tell if this problem is mitigated on those devices. I'd expect at least that it performs better in the background on a 4S, since it should be able to make use of that phone's extra processing power.
Bottom line: For 99 cents, every headphone enthusiast with an iPhone owes it to himself or herself to check out Headquake. You may not fall in love with it, but you'll appreciate experiencing it. And you'll probably keep it around at least for those songs where it absolutely shines.
