If your problem is with inconsistently-encoded MP3s, you’d be better off fixing the gain on those files witih mp3gain. Its software to set all your music at a standard gain. I suggest you target them at 92dB rather than the default 89 (which is rather quiet).
Just Google for it.
If you really want a cheap headphone amp, look for the Boostaroo. You can often get one cheap on eBay. Its also useful for using multiple headphones on one device (without losing volume).
