I happen to think the stock Russian tubes sound VERY good in the Chinook. But if I was going to play around with it I'd try either the Simens E88CC gold-pin tubes or the Amperex gold-pin 7308's if you can get them.
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I tend to agree with Plato. I replaced the stock Electro Harmonix tubes with expensive NOS Siemens Halske tubes from the 1960s. While they sounded good, the EH tubes sounded better overall...go figure. I think the Chinook is "tuned" to the EH tubes when it leaves the factory. Other tubes may produce a different result, but I'm currently using the EH tubes with complete satisfaction. Just my $0.02.