Before you get into buying an amp, maybe you should try boosting the bass with EQ? Unless you really feel like the PSB M4U1 is lacking power to feel more lively. The FiiO E6 is a really tiny amp which shouldn't do much with full size IMO. Personally, I am using E07K (similar SQ vs E6 in amp section) which I uses it as DAC only unless I wish to have more warmth and a little bit of power for some of my headphone.
Just get the E12/E17 if you wish to see a clearer upgrade
the E12/E17 will benefit your future as you do not have to worry no more if you wish to upgrade to something like Beyerdynamic, AKG k7xx. The E12 have very high output power with very low output impedance, it would be a better choice to go for it then you will no longer have to worry about your amp's capability
Hope it helps