Sound quality is going to be about the same using the internal amp using external amps and new speakers would allow much better quality.
The nice thing about the alpine is if you do not want to add a big external amp and new wiring and all you can use there add on amp.
All you do is plug the wiring harness into it than to your alpine head unit and your done. You just added 45 watts RMS to your system.
Alpine KTP-445 Plug and Play Head Unit Power Pack for Use with all 2005-Up Alpine Head Units
I would just install new speakers if I were oing to use that.
Internal amps running stock speakers is going to sound better than the stock one but is going to sound about the same compared to each other.