'ART' is an acronym for Anniversary Reference Triode while 'ACT' stands for Advanced Composite Triode.
True to its name, the C-J ART III commemorates Conrad-Johnson's Thirty Year Anniversary. It is an all-out cost-be-damned assault on the state of the art. Only 25 ART IIIs will be made at $25k each, versus 250 limited edition prior release ARTs, version one of which was issued in 1997 on C-Js Twentieth Anniversary at a cost of $16k. All ARTs subsequent to the original and all ACT2s - nay, all Premier level C-J preamps following the first ART (Premiers 16,17, and CT5) - are inheritors of C-Js seminal composite triode circuit design. The ACT2 ($13.5k), ACT2.2 ($16.5k), and ART III reveal ever increasing use of C-Js own very expensive Teflon capacitors.
Downunder and I have a running disagreement about the ACT series as he prefers a sound warmer than neutral. I'll defer you to both my own
and Roy Gregory's HiFi+
reviews of versions of the ACT2 for a thorough accounting. The ACT2 and 2.2 are advancements in virtually all respects (internal componentry and sonics) to the original ART although there is a difference in their harmonic balance relative to the yin and the yang. Suffice it to say, the entire ART/ACT lineup represents some of the Very Best preamps made by anyone in the last 10 years.
Prior to C-J's Thirtieth Anniversary, the ACT2.2 was their top shelf preamp. And if all ART IIIs are sold tomorrow (if they are not already) the 2.2 resumes that position.