Normally I would say get the 200x2. But since your Yammi is distorting the highs at -6 and since you bought it refurbished, I am sure something needs fixing. So I wouldn't rely on it and I'd move on to the LPA-1 multi-channel.

Alternatively, you can get the 200Wx2 now and live with your Yammy for your centre and surrounds until you are ready to upgrade the receiver.