Audio confusion !!HELP!! im a noob?

About a year ago i had a sub and amp installed into my 1999 jeep wrangler. i wanted a better bass but nothing that slammed so hard it rattled my jeep apart. they put this together for me.( a Kenwood KAC-5204 amp and a Memphis 12 inch sub 15-PR12S4) i ended up liking it alot. a few weeks ago i finally decided on getting 6.5 inch speakers in my soundbar to replace the 4.5 inch ones, because i was lacking the mid bass( the stuff the sub would not pick up on) . i also wanted it to be louder because with he top off you have so much more room for the sound to escape. i ended up getting Memphis 15-PR62v2 speakers, well my head unit does not seem to have a enough power to play them loud. they are getting distorted when i turn the volume up ( not really even that high) i have a JVC kdr-210 head unit . so now im wondering how do i get my power to those speakers? the whole system does not have to sound the greatest with pitch and tone. being in a jeep, i just want some loud with bass music. can i connect the speakers to my amp as well? do i need a different amp for the whole system? a separate amp for the speakers? a different head unit? and i my end up upgrading my front speakers also someday. im not looking to spend alot more money and may buy used if i really do need to get another amp.:( just trying to see what i can do with what i have. seeing what i may need and so on. thanks for any and all help.:cool: