66 all the way!
I cannot comment on valve amps as I haven't tried them with my 66s but they play nicely with whatever Class AB solid-state amplifier I've thrown at them, whether it be a vintage Yamaha CR-1000, CR-2020, Pioneer SX-1250, etc, or a modern Yamaha A-S3000.
They are fairly efficient so don't need a ton of power (unless you have a huge room or listen at headbanging volumes, if you do then I'd probably recommend another speaker because the HF2000 and MF500 drive units aren't indestructible). They do however go down to 4 ohms in the bass so you need an amp that's happy with a low impedance.
A valve amp with high output impedance (i.e. low damping factor) probably isn't the best match given the passive bass radiator as it may result in a 'slow' sounding bass, but again I don't have the experience of such a pairing to be in a position to either confirm or deny.
Hope this helps.
PS - The 44s are nice speakers too, very lush and easy-going, but sound too 'cuddly' and 'bloomy' for my tastes.