I'm not from the us and bought a car that came with a stock cd head unit, but its not din o 2din and to get the installation kit i have to import it from japan for 120 or so, the question is if an amplifer with speaker level inputs will sound good or if its a better choice to get the installation kit and use a pioneer 6900 without an amp i already have it and 2 sets of polk 6.5" db coaxials. The amp i'd be looking at would be an alpine mrp-f250 or the mrp-f450
Stock HUs can have all sorts of equalization and/or crossover circuits built into them...... you can try using the speaker level inputs but it's hit or miss as to whether it will work well.
The right way to do it is to just get the kit and use the RCA inputs from the aftermarket HU.