Adding the CF-50 towers as front speakers will likely be a big improvement to your system's overall sound. They will probably blend much better with your sub woofer than those little satellite speakers could. And yes, using your existing small bookshelf speakers as rear channels can work, while keeping the cost down.
What you probably will notice is the difference between the CF-50 towers and the center channel speaker from your old system. That center speaker will probably not be able to keep up with the towers at higher volumes.
The CF-50s are rated by Energy as having 96 dB sensitivity, and the Take Classics are rated at 88 dB. Sensitivity is usually measured as loudness in dB while driving the speaker with 1 watt while the microphone is 1 meter away. A difference of 8 dB is very large, as most people can hear a 3 dB difference (roughly twice as loud). So with front left and right speakers more sensitive than the center speaker, by 8 dB, you will notice that.
As an aside, I find those claimed sensitivity numbers hard to believe. As a guess (I haven't heard them), I would expect those towers to be in the 87-88 dB range, and those little satellite speakers in the 80-83 dB range. But for the sake of this discussion, let's assume Energy exaggerated both sensitivity numbers by the same amount.
So, I agree with sawz, and partially agree with Steve
![Big Grin :D :D](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
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