Re: Bass amp for beginner
I've popped 6" guitar speakers with a bass when turning the amp up loud enough to play along with a pre-recorded piece. If you just need to hear sound while you practice chords, scales, and arpeggios, then yeah, a 20w guitar amp with the volume low will do.
If you're intending to gig, go ahead and get 300w minimum so you're ready for most any situation, as you may not have the luxury of piping into any sort of p.a. for help, especially if the singer owns the p.a. and has all the channels dedicated to himself. Or if the drummer owns the p.a. and runs a mic to each piece, leaving only room for vocals. Or you just may end up at a venue that only has a single-channel p.a. that can't handle bass (hog calling contest/county fair setup).
With a 300w amp, you can turn down if it's too much, but if you're short on power, you can't turn it up loud enough.