25Ω – Zu's new load resistors are easy bolt-on tweaks to reduce their speakers' 16Ω impedance to 8Ω as something many amplifiers are happier with. What's more, the dynamic impedance ratio shrinks from 5:1 to 3:1 to make impedance peaks much shallower, thus easier to drive particularly for SET-type amps including the FirstWatt SIT.  To install, simply unscrew the 5-way binding posts, slide over the posts, screw nuts back on. No tools, takes about a minute. The graph's improvements shown for Soul Superfly apply equally to Druid and Soul Supreme.