Zu speakers don't need high power but they can benefit from it on some music if the amp doesn't give up too much qualitatively to deliver power. I usually listen to 10-18w single-ended pentode and push-pull triode or pentode amplification on both my Zu systems, but keep a pair of solid state monoblocks around (~270w into 6 ohm Def4; 90w into 16 ohm Druid6) for certain uncorked experiences. Of all the Zu speakers, the Soul Supreme will certainly be a different presentation than your Harbeth, but also the Zu closest to it. Soul Supreme is a 16 ohms speaker so your Hegel 390 will see its power reduced to ~125/125w, a considerable cut since the Harbeth is a 6 ohms speaker, raising your amp's potential output to about 375/375w into that load. Still more than you need for Zu but less overkill. You may find it sounds a little cleaner into the Soul Supreme's higher impedance.
The Soul 6 pair I am listening to are nearly settled enough for commentary, so I'll post more in a week or so.
Phil