The "most" bass won't be the most accurate bass. I use 16 inch stands, which are close in height to the 17 inch matching wooden stands that Harbeth used to provide with the speakers. Any lower than that and you may get more bass, but probably some lumpiness. I'd recommend 16-18 inches. Charlie and others, however, use higher stands, and seem to be happy.
To my ears, and confirmed by my in-room measurements taken from a listening position at which higher stands would make sense, the speakers sound thinner in the bass on 21 inch stands than they do on 16 inch stands.
Paul