Pick up a copy of the Orion Blue Book ($150 for a new current copy, used $50-$100). It lists all ARC models, years made, original retails, and basic specs.
ARC product history (Audio Research)
I'd like to learn more about the manufacturing dates and specifications (not to mention reviews) of ARC's product line, going back some years. Their website is great, but only covers the current line-up. Are there any ARC enthusiast pages out there? Or other good references?