I have an Alpine 9851 HU. I was wondering, can I just buy Infinity Kappa Components and use those without an amp?

 

Yes you can, but you will not get the full potential of the speakers without more power then the Alpines internal amp.
That said I run an Alpine HU and 4 Eclipse 6.5" 2 ways, and have for some years now, no sub or amps, works for me. 94

 

night and day difference when components are amped. you'll be fine tho just running it off the hu

 

I have an Alpine deck with Alpine Type R 6.5 Components, it is a night and day difference with and without and amp.

 

What amp are you running?

 

Any amp that will push the recommended power to the speakers will be fine, try to find one as close as possible. I would personally get a four channel because then, if it so happens, you would be able to amplify another set of speakers as well. But like everyone said, your HU will push them fine, especially if you've got 8 guage going to the yellow wire on back (which is recommended). IMO
Nate