In case I need to replace the tubes I checked out tubedepot.com to see what they would cost and in the process pulled out the tubes to find out which ones to order if needed. There are two GT-EL84-S tubes but the pre-amp tubes have two GT-12AX7-R tubes and one GT-ECC83-S tube. Are those two models interchangeable? Everything I read says there should be three 12AX7 tubes. What do you guys think?