I recommend Boom as well, increases volume and you can do equalizer... I was like, this does nothing,
turn equalizer off wow, music is so bland.
I have Boom and it's great for the internal speakers but it doesn't change the fact that the audio outputted from the audio out port is poor. Basically, non-boosted but Boom-equalized sound is still much worse than the normal non-equalized sound from an iPhone, for example.
Anyway, I ordered a pre-amp/DAC that uses USB or optical out rather than analog so it should solve my problem.