Feed back through front speakers

I've got and Alpine 9835 HU with a 500/1 amp driving a 12W6. I tried adding a Alpine 420 2 channel amp to the rear speakers, but it made the front speakers distort something awfull, both front and rear were distorted. Till I went into the head unit to shut off its amp, then I got the Alpine to sound good on the rear. Why would if I have the RCA cables in the rear output it would effect the front channel? I would like to get JL's 300/4 or 450/4 in the future. I checked all my grounds and every thing I could think of. Any help I could get would be great.
Thanks
Dave