Best Pre for ATC 100's

May I suggest better power cords and a power regenerator, and those parallel power line filters? Those have a humongous difference in the ATCs. They are very very sensitive to this sort of treatment.
Also, plenty of people I know run ATCs with tube preamps and I read online that YBA has the perfect preamps for them as well as the preamps from ATC's own line.

Don't forget to change the fuses as well. Though you need a professional to do them for you. The fuse holders are crazy hard to disassemble.
And you really should give us more details about what other components you have in your system. In particular, what dac or source are you using?