I mostly agree with Jook.
One option he didn't mention, bass usually works better if you use a line out to the PA instead of a mic. A mic will work, but unlike guitar, bass works very well with a line signal going to the PA. I find guitar amps with a line out usually don't sound as good as a mic in a speaker.
Also, I've been onstage with lots of bass players, 300 watts is plenty. The general rule when I was playing for a living was twice the wattage of the guitar amp, to get a clean bass sound and still have some headroom. It's not common to find many guitar players running 100 watt amps any more, most are going downward to 50 and even 30 watt amps, I'm playing a 15 watt Fender Pro Jr tube amp now most of the time, the 45 watt Fender Super Reverb if we have a high volume gig, which is not often. The Pro Jr is too loud for most of our gigs, I have to keep it down around 4. Our bass player is using an older Peavey 150 watt keyboard amp that does well, he also plays keyboards and uses one for drum machine.
I've played with bands with the bass run into the PA with a line signal several times, it works really well. If your amp has a line out, all you need to buy is another guitar cable. Run it into the PA, problem solved for 20 bucks. If you need onstage volume, that's what good monitors are for. I can do the same with guitar and a mic using the 15 watt amp. Mic the guitar amp, run it back through the monitors. Been there, done that.
Try a line signal into the PA before you spend any money, all you need its a standard guitar cable. If your amp has a line out. If not, a 300 watt amp with a line out is what you need. Your current rig will do the trick, or a different amp that can send a line signal to the PA.
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