I'm a guitarist who is now playing bass for my current band and i need an amp. I only have a budget of about 500 dollars though. I need something capable of playing medium sized gigs. I don't know if I should go for a combo amp, or buy a head and cab. Any suggestions would be appreciated.
Also, I have a cheap 4x12 guitar cab I'm not using. Would that work with a bass head? Or is that a silly question? I really just don't know lol.
You'll get better responses in the Bass Forum! I'd get a proper bass cab .. when you start grooving, you'll automatically start turning the volume higher, not good for your guitar cab.

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Look for some Peavey, it's good quality for the money. I wouldnt recommend playing bass through a guitar cab, if you push it too hard (which will be sooner than you think) it will be dead in no time, also it won't sound nice.
Don't use guitar speakers.

What's the gigging situation like: how much volume are you going to need?

Used peavey and carvin gear is abundant, cheap, super tough, and sounds alright. Probably the best bang for your budget if you're picking up a 4*10 and a head.

With your budget, you might also be able to get a decent used GK head (stay far away from their budget line models though). You also want to avoid the cheaper fender and ampeg stuff. Anything worth having from those companies (other than some ampeg cabs) is going to be out of your budget even used.

If you only need monitor volume and not an insane amount of it, you can go the combo route no problem, but you'll probably want one that can tilt or buy a tilted floor amp stand. Again, used Peavey and Carvin are good choices.

If you have to go new for some reason, your options go very far down hill very quickly.
You can get a small (200W is small for bass) SWR combo in your pricerange too. SWR bass rigs rock.
Gilchrist custom
Yamaha SBG500
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100

Cathbard Amplification
My band