I'm buying a bass... can I play it through my Line 6 guitar amp? Or do I need to buy a new amp?

you can, but it will sound crappy and I dont think it would be good for your amp
you can
get a bass amp
or one day
youre gna be jamming to some song you REALLY like and put the volume way up

and your amp will explode.

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add that:
It will slowly destroy the speakers. Just get a cheap bass amp. It will sound far better (you will have some low end) and it will have speakers built to handle low frequencies.
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Dude, just get a bass starter pack. Comes with like a 15 watt bass amp. Shouldn't be more than your average starter bass.

Get an Ibanez or a Squier pack.