ATC SCA-2.... You cannot go wrong pairing with their own pre amplifier and is very cost effective. I have even used the SCA-2 with a pair of SCM-300ASL.
If you're going to use a valve pre amplifier then the AN M8 is a good choice but I personally would use a solid state pre amplifier with the ATC speakers. The suggested Spectral 30SS is a great contender as well as the Krell Evo-202 or Evo-2. You can also try the MSB DAC-4 Diamond with a volume pod to drive direct.