6.5 component set under $400 - which one?

Many components can handle that kind of power if used with a high pass crossover.  Do you currently have one on your Clarions?  What do you feel is lacking from your Clarions?

Tough question.  Many folks obviously are going to say the components they own are the number one choice.  (That's why they bought them!)  Also, nearing the $400 mark you're not going to find many BAD speakers. Not to sound cheesy, but a lot of it at that point is just personal preference. 

I've been personally impressed with Mb Quart, obviously, Diamond Hex, Memphis's higher end line, Kappa Perfects, Focal's and Boston Acoustics.  Whether they are $400 or under depends on whether you buy online or not. 

I'd also like to hear the Koda 6 components from Adire and the REaudio.com offerings.

First, what amp are you currently using to power your Clarions?

Second, I'm not an installer, but I picked the Mb Quart PCE 216 set for myself because they were the most crisp and clear speakers through the midrange and highs that I demoed.  They also handle quite a bit of power, but don't need a lot to get loud.  The midbass is slightly lacking compared to some of the offerings from other brands, but I value vocal midrange over midbass for my mains.  I also need accurate highs because I listen to quite a bit of violin music.  Some folks don't like how crisp the quart tweeters are, saying they are two bright.  You'll have to hear them and see what you think.  Another huge plus for me was the ability to install the tweeter in the coaxial position.  I didn't want to find somewhere else to mount it in my car at the time. 

I don't think you can go wrong with the Quart PCE set.  You can find them for around $300 shipped online or though a dealer willing to push some inventory off near cost.

Having said that, I don't think you can go wrong with many sets in this price range.  They all have their strengths and weaknesses.