sometimes when u use a bass on a guitar amp you will get some unwanted voices (not pontianak voice) like reverb etc. it doesnt really work on the bass. the bass is not just a guitar tuned an octave down, theres alot of tonal differences as well
i have a bass and a crappy squire bullet strat and i only have a bass amp. the amp makes it sound fatter. but doesnt really work for metal, or playing chords. gets too muddy. but then i hardly play metal, or guitar for that matter