The H/K 6x9s are 2 ohms, if memory serves, which is what the BMW H/K amp is expecting to see. Your typical aftermarket car speakers are 4 ohms impedance.
If you hook up a 4 ohm speaker to your H/K amp, with the doubled impedance, your speaker will only see about 12-15 watts of clean (less than 5% THD) power from the amp, so it really doesn't matter what you pop back there- they will be running at well below their power capacity.
As for audio quality, these are rear fill speakers, and the amp has a built-in cross-over which will send the woofers ONLY mid/ low frequencies. You need therefore be concerned with only three things; efficiency (how loud they go for a given amount of input), size (you need a 6x9), and cost (as little as possible).
SOLD- Now Enjoying Honda Civic