I've found that speakers in the CR-V are fairly cheap.
I'd recommend looking into replacing all of your speakers with better speakers, then after you've heard them with your existing radio you can then consider a replacement for it if you're still not satisfied.
Since you are asking about a CR-V with a CTDi engine I know you're not in the US, so I don't have a suggestion as to where to buy better speakers. Good luck.
Many thanks Racoon for your advice. If I put sound onto C position ( for balance & bass/treble ) then RHR speaker doesn't work . If I put all sound to rear speakers , I get nothing at all .... John Rhodes