ARRL did a rather thorough test of a number of commercial tuners several years ago. They tested them on 160-10m and with loads well below and above 50 ohms. I recall that several were extremely lossy on 160 with severe loads. The article is on the ARRL web site and can be found with a search if you are an ARRL member. I don't have the link here. 73, Dan, N5AR