I think it very unlikely that for £50 you will get anything to noticeably improve upon the Cowon’s internal amplifier. These units are well known to sound good and don’t need any external amplification.
However, for some three or so years now I have been using a MiniBox E with my Cowon D2 and have found that it improved the dynamics of the Cowon player when used with my Grado SR125s or Senn 650s. The only problem is that the MiniBox is way above your budget.
The MiniBox brings a subtle improvement when listening to acoustic instruments (not processed pop music) in that the sound without it is perfectly fine but with the amp there is a perceptible improvement in dynamics and tangability in that music sounds more like real instruments playing. (This is particularly noticable with my own live recordings.)
It’s difficult to explain why but I suspect that a separate amplifier with its own set of batteries has better control of the headphone diaphragms. It reminds me of listening to a modern high end amp with a stiff PSU compared to say a classic valve amp, in that the modern amp has more ’grunt’ (though of course might lose out to the classic amp in other terms).
In other words, don’t waste your money.
