if u have a vitz 08, that would mean ur car already has components upfront. this amp of urz is 2-channel and the pioneer speakers that u have are actually pretty good unless theyr either fake or damaged (in which case they wont sound good)
now u may sell the kenwood and/or the pioneer speakers too.
if budget isnt an issue go for
1) Pioneer 6975v2.......11k
2) Pioneer 8604..........14k................ this is a 4 channel amplifier, u can connect both ur front speakers and rear speakers to this amp
if budget is an issue go for
1) Pioneer 6975S........ 4600
2) Pioneer gm6500f..... 8500............ this is the older version of the 8604 amplifier.
else u can look around and check some of the soundstream tarantula and kenwood/sony amps. remember get a 4-channel. as for the speakers look for some jbl and infinity 6x9s aswell.