Assumed-And is the way Google does it, with the more search terms added, the narrower the results. The other way around can be argued in the abstract, but your customers are not living in the abstract. The world has voted, and Assumed-And is the way it is. Having additional terms widen, rather than narrow, the scope confuses people in the extreme. They will leave you and find a site with a search function that “works.” This blunder alone could put a company out of business.