So basically what you need is a 6×9 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_1773…
Most of the 6×9’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 6×9’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.