With all The Audio Research power amps and most all other power amps that I have been using over the last 15 years, when replacing any of the tubes the amp needs to be rebiased, even with the use of the same tube (6550's, KT88's EL34 etc).
One does not have to buy replacement tubes from the manufacture of the amp to get good tubes, such as the Sveltlana, Sovtek, there are very good suppliers on the NET, that will supply you with tubes and in most cases at much cheaper cost to you, but the amp WILL need to be rebiased after replacing the tubes. If you don't want to attempt this yourself most any High End Audio Store should be able to complete this, you shouldn't have to send it back to Audio Research. Good Luck.....
One does not have to buy replacement tubes from the manufacture of the amp to get good tubes, such as the Sveltlana, Sovtek, there are very good suppliers on the NET, that will supply you with tubes and in most cases at much cheaper cost to you, but the amp WILL need to be rebiased after replacing the tubes. If you don't want to attempt this yourself most any High End Audio Store should be able to complete this, you shouldn't have to send it back to Audio Research. Good Luck.....