help setting up my home system


hey guys, at home i have this old yamaha rx v357 receiver, and few years back I did not know so much about sound systems. So I connected it to edirol UA 4fx, 

so my system set-up is

1- destop windows
2- connected to the yamaha rx v357 via optical cable
3- the yamaha is also connected via optical to the edirol via optical

it is been so long since I added anything new to the system, and I think the edirol is more for music recording than listening but I did not know it back then.

I have 2 advent prestige 75 speakers (6 ohms, 175w, 65hz to 20Khz ), also few days back I got a good offer to buy only one cerwin vega VE12 (300 watts, 28hz-20Khz)  speaker  and I am not knowing what is best to remove one of the 2 advent speakers and connect the cerwin instead of it, but will this lead to unbalanced sound, as the cerwin is way stronger than the other 2 speakers.

In general would you buy a new system or add anything new to the system.

I am sure I will not get another cerwin vega for cheap , so any recommendation in setting this up properly is appreciated