So basically what you need is a 6x9 that performs well at low power.
I have always been impressed with Infinity/JBL as far as low power efficiency.
Even though these can handle a lot of power - I would expect them to work well with just the head unit running them:
http://www.sonicelectronix.com/item_17734_JBL-GTO928-6-x-9-Coaxial-Car-Speakers.html
Most of the 6x9's from the recognizable brands should perform reasonably well. To make the best decision you should just shop some local stores. Ask to hear as many 6x9's as you can on just HU power. That's the only way to make a truly informed decision. Only you know what sounds good to you.