Have you tried Audio Research Corporation. www.audioresearch.com There phone number is 763-577-9700. Another possibility is Lenbrook Industries, phone# 781-784-8586. They did a v.4 upgrade on my ARC Ref. 1.
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I bought my REF 1 used. it came with the infiniti caps, detachable power cord, gain control adjustment, etc. already installed. In the owners manual was a service sheet from Lenbrook Industries indicating a v.4 upgrade. If you want better details, you should contact Leonard at Audio Research or Lenbrook Industries. They'll be a lot more specific. I upgraded from a ARC LS16 to the REF 1. It was an amazing difference in the overall sound quality, dynamics, soundstage, and detail. I added a Shunyata Viper power cord and NBS Monitor I XLR interconnects, and man am I one happy camper!!!