Stumbled across this app thanks to this post
In which Viper4Android is mentioned almost as an afterthought, with a vague explanation of it making things sound "clearer". The description given made it sound like a gimmick, but upon further exploration...
I was floored by what the app promised to do--and on testing it out I find that it pretty much delivers everything it promised and more!
1. It is a system sound processing module--intercepts all system sounds (apart from phone calls). I confirmed this works with, e.g. Poweramp and KKBOX (streaming music service)
2. It is NOT just like a wrapper for MusicFX, unlike dozens of wannabe system DSP apps. A brief read of the feature list should convince you of that. And I haven't tried them all, but I can certainly confirm that there is a working 10 band EQ (unlike the 5 band of MusicFX).
3. Most importantly, there is a Convolver function, which lets you take a recording of almost any sound processing effect as an impulse response (there are dozens of stock effects but I think it's better for you to record effects yourself) and use it on your phone.
People have been using this feature to record the various bass boost, EQ and spatialization effects of different players and devices (the spatialization effects probably with limited success, since the convolver doesn't yet support cross-channel processing), but as seen in this post
and the OP's response following it, I think the greatest potential of this feature is headphone compensation EQ a la Accudio. You can take the headphone measurements of various headphones from various sites like goldenears, take their frequency response relative to the site's target curve, and apply the inverse EQ with arbitrary level of precision using your EQ of choice (I choose Electri-Q), tweak the resulting curve to taste (e.g. I apply a subbass boost below 100Hz to compensate for the loss of bass in noisy environments), record an impulse response with this EQ, and put the result into Viper4Android and your phone will be able to play music through the same precision crafted corrective EQ that you have on your computer.