I have a passive bass and a Fender Frontman 15g guitar amp. Is it safe to use the amp with my bass?
The amp has 15W and an 8" speaker. Problem with that is that an 8" speaker cannot move a lot of air before it has to turn around. Meaning that it may be overtaxed with reproducing low notes.
Most guitar amps are comparatively robust against overdriving since that's what people tend to do a lot. If most of your notes are low notes, the speaker may get a lot of power while moving comparatively slow, meaning that it may not get rid of the accumulating heat as much as it is designed for.
So probably the most strenuous you can be for a single-speaker guitar amp with a bass is by playing consistently clean low notes near to clipping volume. While possibly cranking up the bass tone control to get more of a punch in your stomach area.
If instead you are aiming for a funky, treble rich sound, possibly with distortion and while leaving the bass tone control alone and forget about the kind of low sound you feel more than hear, I doubt that you'll be damaging anything.
You just might end up more underwhelmed with the amp's power than you would with a guitar.
As long as you're not expecting it to sound like a loud bass amp., yes, it'll be fine. Obviously you're not going to take it to gigs, but certainly for home or garage practice it'll be fine. Just make sure it doesn't get loud enough to distort - as at that point, it may well not be the amp's inbuilt distortion, but the speaker starting to break up! I'd be looking to add an extension speaker to it - 10" minimum, and watch the impedance, as that will improve the sound no end.