I have tried the iBasso PB1 Toucan, BassBoost CMOY, E7, E9. e7 and e9 does well but they arent portable. the best thing ive used is the FiiO E11 paired with a Sansa Clip+, with the Sansa's bass EQ for the lowest band (bass) at half way up. Paired with the E11 with high current mode (under the battery theres a switch), and the E11's bass boost at 2, low gain and volume cranked, i get beautiful bass (no distortion).
But if you aren't interested in in-your-face bass, then really any amp will do. but otherwise the bass will sound quality but far away. Honestly after the FiiO E11 i can finally use my DT770s 80ohm for music and not just gaming. but understand everyone has their own preference, im just a basshead.