I have Classics and have driven them with Audio Valve monoblocks(Dynaco MK IV) modded to run in triode at 20 watts. No problem unless you like it really loud.
I normally run mine with Manley monoblocks in triode mode at 60 watts.
I would say 30-40 watts would give you all you need for non-extreme listening.
I have even run them an with 8 watt 300b amp. They handled it well if I stayed away from rock and orchestral. Light jazz and acoustic was fine at moderate volumes.