Here is the page on SlowTrav that will be most helpful. The reviews of rentals in Tuscany are divided into different regions, which helps a lot. I think the area of southern Tuscany is perhaps the most interesting: Montalcino, Pienza, Montepulciano, etc. Vacanza Bella is a terrific company - its owner, Daniel Morneau, is very knowledgeable. Another great company is Tuscan House - it has a lot of beautiful properties in the Siena region. And yes, I've heard of Untours. We have friends who have used them for a couple of trips and have been very pleased. Personally, though, I think it's easy to do it yourself, especially with the help of SlowTrav and its forum, SlowTalk.