Page 1 of Sub woofer & speakers question

I recently upgraded my amp to a Technics 5.1 (from pro-logic) I have Eltax Milennium 500`s front & Wharfedale Valdus at the rear. These are fine for me they sound as good as I need, but, I`m looking at adding a centre & sub woofer now I have a reasonable amp. Daft question alert! - is anything to be gained sound wise by adding a centre & sub woofer. More importantly given the size of my front speakers, does the sub need to be larger than them?
This may be a stupid question I know but up until now I`ve used `phantom mode` across the front and have pretty good bass reproduction already. Having said that I haven`t heard a proper 5.1 set-up that has anything other than coke can sized speakers front & rear plus a sub!

Firstly a front centre is a must,it will allow your system to feed the audio to each speaker independently giving you dolby digital or dts as it should be.

This will not only improve the dialogue no end,but the whole sound field will be dramatically improved.

As a rule you should always try to match speakers,so try to get the matching centre to your front`s.

As for bass this is up to you,but it would allow you to here the low frequencies without having high volume alround.Plus you would be hearing the audio as near to the original as your system would allow.

Hope this help`s.