So I installed my new Infinity Kappa 6x9 3 ways in the rear of my car the other day, they sound awesome. When I turn the bass up, it sounds great, but once I turn the volume up with the bass, everything aside from the bass starts getting distorted. Is this because I'm running them off of my head unit and there is a lack of power to the speakers? I'm hoping this is the case because I had planned on getting an amp sometime soon. Also would the Infinity Reference 475a be a good amp for my setup? I have the Kappa 3 ways in the back and the 4x6 plate kappas in the front. Thanks.

yes...get a good amp for those puppys...thats most likely the reason they:re doing that..your just underpowering them..no biggie..

That indeed sounds like you are underpowering them.
According to what Ive read, your HU's internal amp might be pushing itself to its limits generating DC,which causes distortions in your speakers and will eventually blow them.

Speakers Are blown more often from underpowering than overpowering

does sound like amp overload-i like to run my multi-
element speakers in a "bi amp" configuration,that is
running the woofer and the tweeter/midrange off two
seperate amps,some speakers are designed so this can
be done if desired,others need modification.the main
benefit is with seperate amp,the current pulled by the
woofer cannot modulate and distort the tweeter/mids
output. on my dodge,i have the tweeter/mids of my
pioneer 6x9s running of the pioneer head's internal amp
with the woofers running off a 60w orion amp-sounds
real decent for a simple "work beater" system.
(but nowhere near as good as my camaro system)