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.