I've done quite a few tours with "Finantur" who are on Madame Lynch, between M. Lopez and Dr E. Ayala, 4 or 5 blocks from M. Lopez, opposite a service station and love motel. They are local and everything is in Spanish, and they specialise in historical tours, based around the War of Triple Alliance. I have done the Chaco, Cerro Cora, Pilar, Paso de Patria, the Rosada/La Colmena area and a 6 day tour to Corumba in Brazil, .
I will say that they are very enthusiastic and friendly, but sometimes the buses they hire are on the small side for me, but I am 187cm and 125kg. As tourism in Paraguay is a newish thing they are learning as they go, but to me it all part of the Paraguay experience, and its good to see people with such enthusiasm. You will also meet some great locals, as the tours, while not expensive, are well above the level of most of the populous, so everyone will be educated and willing to share.
I keep going back, and in fact I returned from Salta to Asuncion to do the Brazil trip, so they must be doing something right.
Tell them "Senor Michael from Australia" recommended them. It won't save you any money but you will become an instant friend.