From the Vault

Inside the Reference 600 power amps are the eight matched pairs of 6550C output tubes split into two sets on facing sides. The rest of the valve complement consists of another quartet of 6550Cs acting as regulators, two 7233 regulators, two 6922 regulator amplifiers, eight 12AX7 drivers and two 6922 input tubes. Yep: 34 tubes in all, and you're gonna want factory replacements rather than any old stuff, eh? Still, Audio Research has built in an amount of flexibility, so the fastidious user can experiment with 6550s, 6550B, the aforementioned 6550C, KT88s (you wish...), KT90, KT91 or KT100. Note that the company's 'current choice for reliability and long service' is the 'Russian 6550C', with 2000 hours being a conservative estimate for tube life. What I'd love to hear is the impossible: a pair of 600s stuffed with genuine, M-O Valve KT88s...