I just bought a Apline head unit. cda - 7893. Says it has 27 watts rms power x 4 with a 60 watt max. I have 2 blaupunkt 6x9's in the back with 75 w rms each. The two front speakers are alpine 4x6's with 25 w rms i believe. If I was to get two 5 and 1/4 inch speakers for the door panels, would it be necessary to get an amp for my system?

 

Only if you want to use all 6 speakers. If you intend to replace the 4x6's with the 5.25's then no, you won't need an amp. If you try connecting the 5.25's in parallel with the 4x6's you'll end up with a 2ohm load and I haven't seen an HU yet than can power a 2ohm load.