I have heard both and have used a Viva Solista for 5 years. That is one of the best amps I have heard, but I am sure Wavac is in another league. I spent a long time at a show in the UK and felt the sound in the wavac room, was the best I have heard before and since.
having said that, Wavac is another league in price too. If you consider you can pick up a second hand Solista for $4000. I am not sure where you can get a better sound for the price, assuming you have sensitive enough speakers. They do not have to be too sensitive though. I found they had better control of 89db speakers than the 180 watt Karan 180, I was also using at the time.