When a 300 watt bass amp "isn't enough", the situation usually isn't that complex. One or both of the following things are almost certainly true:
1) You have a guitarist (or two) who plays too loud, in part because his speakers don't aim at his ears - they aim at the backs of his knees. But you, being further away, are in the beam, and it's way too loud at your position. So, you turn up, but....
2) Your amp is similarly mis-pointed - other people may be in the beam of your speakers, but you aren't. You can't hear yourself because you're not hearing your rig at full level - it's loud enough, just not at your ears. And you're being drowned out by the guitars - they have a natural "advantage", in that their instruments don't need as much power to be deafening, and you're caught in their crosshairs. You need to get out of this silly situation.
It's time to get creative - figure out what you need to do to point your amps at your own ears, not at each others' ears. If your guitarist(s) refuses to go along, you can either quit the band, or decide to stay and go deaf.
The last thing you need is a louder rig - that's one more step towards insanity; it will just up the volume war, and it will make you deaf faster.