Hello, I was wondering if someone could tell me the sound signature of the Alo Audio Pan Am amplifier, is it a dark sounding bassy amp or a thin sounding amp with extended highs? And what kind of headphone is recommended to use with it, a dark, thin or a neutral sounding can? Also, is it worth getting either the Passport or Getaway external battery packs? Do they improve the sound quality of the amp? Thank You!