the KTP-445 is not an external amp, it's the add-on package for Alpine head units. it's slightly better than a head unit but not as good as a true external amp. external amps are more expensive and the installation is more involved.
if you didn't get enough output from 6x9's, going smaller to 6.5" won't help. there are better speakers suggested, but they won't be louder and they will have less bass output.
the OP needs to recognize that with the KTP-445 he only has so much power available, and Vol 20-25 should be considered a max setting on the head unit. also, bass boost settings should be minimal. if more bass is desired, an external amp/sub combo is the next step.
blowing speakers can be done with a KTP-445 if your bass boost settings are too high and your gain is on High.
how did you verify your speakers are blown? did they just stop working or did you test them? you could simply have a wiring problem which is causing channels to cut out. i would test the speakers with a DMM first, and if they meter out ok (3.4 - 3.8 ohms ) next test with a separate amp or home receiver. if the speakers are fine, your wiring needs work.
if the speakers work - describe to us how you made the connections, even share pics of them.
to make recommendations, a budget is needed. Even Alpine Type-R are better than what you had.