check out their website,www.audioresearch.com, they will tell you about the upgrade.I own mkIIs,best I,ve ever heard.They are big, use much power,and throw off alot of heat. You need a large room,but it,s worth it.
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Directly from Audio Research: There is good news for REFERENCE 600 owners. The same sonic improvements found in the new REF300 can be obtained in a Mark II Factory Update program for the REF600. Original REF600 units are literally taken apart and painstakingly rebuilt with new output circuit boards and new input / regulator board, using all-new components. The rear panel is also retrofitted with a new three-pin, 20-amp A.C. connector and detachable cord. At the same time, the convenience and flexibility of the original REF600 has been preserved. Front-panel controls and large, illuminated analog meters allow easy output-tube biasing, A.C. line voltage monitoring, and power-output observation. Using all vacuum-tube input and driver circuitry (6922 twin triodes), as well as tube regulation, each REF600 MKII employs eight matched pair of type 6550-C output tubes to yield well over 500 watts of wide-bandwidth power into speaker impedances ranging from 1 to 16 ohms. Main power supply energy storage is rated at 669 joules. Output terminals are proprietary gold-plated 5-way binding posts.