I would think carefully before changing from SS to tubes for the Harbeth 40.2's.
I recently bought the 40.3's and even with gobs of power my Rogers High Fidelity EHF-200 did not sound good with the big Harbeth's.
It's not about power it's about damping factor/impedance matching.
I switched to a BAT VK3500 (hybrid tube/SS) and they sound amazing, but this was an expensive lesson for me.