Have you tried the Naked Zero autotransformers between your tube amps and the SLs?
<https://anticables.com/store/%E2%80%9CNaked%E2%80%9D-ZERO-Autoformers-p14644819>
They help with impedance matching to transfer power more efficiently and they will enhance the SQ without adding distortion. Ralph at Atma-Sphere recommended them to me. They work great. I have even been able to drive SLs with Atma-Sphere M60s (60-watts tube mono-block amps}