What Speakers?

[Niall]

Focal's would be my recommendation, they're what I run in my cars. Although you'll see a much better improvement upgrading your head unit too. It's all well and good changing the speakers, but it won't help that much if the source isn't up to the job. In my experience Sony are fairly rubbish with HUs. Get a second hand Alpine off of eBay and you'll hear the difference. I know you said you don't want an amp, but that's what you need to get the best from the speakers. It's not expensive, nor is it hard to do.

This. Unfortunately getting decent quality audio is a slippery slope. Ive had 2 Sony HUs now and both have been shockingly bad in comparison to other brands.
Also, when you are going for a mid range speaker like them Focals, they will sound fine but only when you put them on a amp do they really come to life! I was shocked at the difference in quality of sound when i amped mine. Was like going and buying new speakers.