ok so me and my friends are starting a band im going to play bass guitar which i already have but the thing is i dont have a bass amp. My band will be playing very heavy punk such as grunge and are going to jump straight into nirvana covers our guitarist has a valveking 112 50w amp with a ds-2 pedal. I just need suggestions on what kind of amp i should get that can keep up with a 50w tube amp and a very agressive drummer.

P.S im on a budget looking for anything around 200-400$ and when you suggest an amp will you give me a reason for your suggestion and any other feedback on what to advoid.
Have you looked at the GK Backline series? They aren't great, but are about the best you'll get in that price range.
ESP LTD EC-256 and a Fender Deluxe VM
You can probably get an Ashdown MAG210 for that or a Peavey TNT115...they'd be my choices in that price range.