What makes a speaker tube friendly?

Many of our customers are tube enthusiasts (or aspiring tube enthusiasts), hence one of the most common questions we are asked is "Will my speakers run on tubes?" Many consumers are confused, and others clearly misinformed, regarding what characteristics of loudspeakers yield a "tube friendly" design. We have been unable to find a good, concise discussion of these characteristics online and as a result have decided to write one ourselves. This article will provide a basic explanation as to why some speakers work well with tube amplifiers while some do not.